It's not a "slight difference", it's a real difference, because you are using two different commands (a DO and an IF command)

set a = -1        // sets the variable a to -1
do $increment(a,1) zw "test" // increments <a> by 1, <a> is now 0
                             // the $increment() returns (the value of <a>)
                             // but the DO command do not expects a return value
                             // so the returned 0 is disposed
                             // hereinafter the next command on the line (zw) is executed
if $increment(a,1) zw "test" // increments <a> by 1, <a> is now 0
                             // the $increment() returns (the value of <a>) which is 0
                             // the IF command takes the returned value (0) but the value is
                             // "FALSE", hence the rest of the line is skipped (old style IF)
                             

If yo want to use the IF command and want both commands to be executed and have them on one line then:

// you have several options, a few of them
//
if $increment(a,1) { } zw "test" // empty IF-statement
if $increment(a,1)!1 zw "test"   // an IF with a forced outcome
if $increment(a,1)=a zw "test"   // if a=$increment(a,1) WON'T WORK !!!
//
// The only limit is only your imagination...

By the way, it's not a good idea using commands/functions with abbrevations and at the same time using mixed cases. In your examples I had to take a second look to realize, the one is a capital "i" and not a lowercase "L"

if $I(a,1) // increment a by 1
if $l(a,1) // return the subfieldcount from <a>, delimited by the character "1"

This works for me (pay attention to the semicolons!)

ClassMethod numpytest() As %String [ Language = python ]
{
 import numpy as np
 import math
 arr = np.array([1, 2, 3, 4, 5]); print(arr); x=np.random.randint(1, 15, 1) # generate a number between 1 and 15
 print("Random num=",x)
 y=math.pi
 print("This is pi:",y)
 x="hello world"
 print(x)
 a=2+5
 print("a=",a)
}

an the output is

USER>d ##class(Py.Demo).numpytest()
[1 2 3 4 5]
Random num= [13]
This is pi: 3.141592653589793
hello world
a= 7

USER>w $zv
IRIS for UNIX (Ubuntu Server LTS for x86-64) 2021.2 (Build 649U) Thu Jan 20 2022 08:49:51 EST
USER>

Just a small hint, if you have more than one statement on one line, separate them with a semicolon

print('one'); print('two') # this is OK
print('one') print('two')  # syntax error

On IRIS, long strings are enabled per default, you can check that

set x="", $e(x,3641144)="z"  // OK
set x="", $e(x,3641145)="z"  // <MAXSTRING> error

Just to see things clear, the size of a stream is limited by the size of the database (some GBs or even TBs) and the size of the storage media (hdd,sdd), whichever smaller is (less the size of other globals). The size of a table row (all fields together) is limited by the string size (either 32KB or 3.6MB). In a table row you can put a REFERENCE of a stream but not the stream itself.

Just a wild guess... somewhere some character type checkbox is checked or not checked? Maybe a wrong driver type (ASCII/Unicode)?

// For example, IRIS sends a username "paul" as a two byte sequence
0x70 0x00 0x61 0x00 0x75 0x00 0x6C 0x00

// the other end sees
0x70 - this is a 'p'
0x00 - C-type end of string (hence, the name is just 'p')

But as I said, it's just a guess...

Can you please ensure, my answer matches the original question? Thanks! Usually one makes a new question instead of changing an old question to a new one. Currently both my answers hang around without an context to the above question.

There arises two questions.

First, why do you use a class where the documentation starts with "This class is used internally to hold property/field values needed for computed fields"?
Another word for classes which are marked as "used internally" is, that this classes can be changed (altered in its behavior) without any notice or documentation in one of the next versions. That's definitelly not what you want.
 
Second, what is your real problem (where you think, it's only solvable by using that %Library.PropertyHelper class)? Can you post (and describe) your real problem?

Two notes to your (above) code

- first, if you use the iterator method on an object which can contain long strings (longer what IRIS can handle) then you must specify the thrid argument to the %GetNext() method too, i.e.

while iterator.%GetNext(.key, .value, "stream") { ... }

- second, if you know the name and location of a stream property in an JSON object, like in your case, then just grab the data without using an iterator:

try { set jsonobj = {}.%FromJSON(httprequest.HttpResponse.Data) } catch { jsonobj=0 }
if jsonobj, jsonobj.statusCode = 200 {
   set pdf=jsonobj.value.labReport.%Get("pdf",,"stream<base64")  // according to the picture you provided
   ... // do something with the pdf-stream
}

For another example, how to work with long strings see this thread.

You have right, do {code} and do {code} while expr are mutually exclusive.

I thought more something like this do {{code}} (no spaces between curly braces) or, as you suggested, a new keyword like WRAP {...}, of course, BLOCK {...} and SCOPE {...} are also acceptable.

By the way, we already have an alternative for argumentless DO
do {code} while 0
It looks ugly and confusing and
- does not preseve $T
- does not establich a new stack level
but one can get over both easily respective do a work-around.

I hope someone is already working on that extension and it's scheduled for the upcoming release

Oh, I see, at the very first use of a (new) private global,  the storage is decremented (on my  machine) by 768 bytes. I think, that bytes will hold some management data for each provate global (name).

Instance: cindy:ICINDY
Version : IRIS for UNIX (Ubuntu Server LTS for x86-64) 2021.2 (Build 649U) Thu Jan 20 2022 08:49:51 EST

Username: kav
Password: ******
USER>s x=$storage

USER>set x=$storage, ^myKav=123 write x-$storage
768
USER>set x=$storage, ^myKav2=123 write x-$storage
768
USER>set x=$storage, ^myKav3=123 write x-$storage
768
USER>set x=$storage, ^myKav3=123 write x-$storage
0
USER>set x=$storage, ^myKav2=123 write x-$storage
0
USER>set x=$storage, ^myKav=123 write x-$storage
0
USER>

I don't see any difference

USER>write $storage,! set ^myTest=123 write $storage
2199022714000
2199022714000
USER>write $zv
IRIS for UNIX (Ubuntu Server LTS for x86-64) 2021.2 (Build 649U) Thu Jan 20 2022 08:49:51 EST
USER>

Try the above line. Maybe you have issued one or more "set variable=..." commands between the two "write $storage" statements in your tests...

Just a curious question, do you talk about a form with encoding? Something like this

<form method="post" enctype="multipart/form-data" ...>
...
</form>

If yes, I think WRC will be your friend. In any case, I have never worked with such type of encoding. Maybe someone else?

And you are certain,  the correct name is "profile"?