Sorry!  That doesn't indicate any reason. $zf(-1) is just a spawn.

using LOG^%ETN gives a full stack trace. without breaking the process
you just have to place it into your code at the critical points.
https://docs.intersystems.com/iris20201/csp/docbook/Doc.View.cls?KEY=GCOS_errors#GCOS_errors_log_etn

Instead of the double $ZF()  I'd suggest using the query in Library class %File.

 
set rs=##class(%ResultSet).%New("%File:FileSet")
do rs.Execute(SDIR,"*",1)
while rs.Next() { write !,rs.GetData(1) }

Let's analyze this:    [leaving aside that $Zf(-1 .. is Deprecated)

1. You run the same sequence in 2 Caché/IRIS  processes in parallel.
2. There is no synchronization between them
3. Both run in the same namespace ==> therefore the same default directory where SDIR_"DIRLIST.TXT" is located ???? " .. but from a different folder... "    is SDIR different ????? if not :
4. Both processes spawn 2 times a sub-process for each $ZF(-1)
5. let's name the sub processes fg1, fg2 and bg1,bg2
6. The sub_process from foreground is not synchronized to the sub-process of background and your construct has no control on the sequence they are running.
7. If the run sequence  fg, fg1,fg2,bg,bg1,bg2  there should be no problem. But that's rater unlikely
8. it looks like fg,bg,fg1,fg2,bg1 (partially deleting results of fg2), bg2  

To verify what's really happening run fg,fg1,fg1,bg (JOB ), bg1,bg2   
So they both can't influence each other.

You may use this at init time  
https://openexchange.intersystems.com/package/helper-for-objectscript-language-extensions   
to add your ZPM command and function. It does a unique insert of the label

@Vic Sun. your reaction is - to formulate it politely - a disappointment.
But no surprise to me.

I understood that 14 licenses + Grace Period are your biggest pains.
In past, I  decreased the risk by moving ALL (suspicious) connections to a dedicated  User
observing his maxConnection limit using %CSP.Session.Login() especially this explanation:

This can't avoid a DDoS attack but it limits the initial impact and allows you to
protect some emergency licenses.

[As I know the inventor of Grace Period since it was rolled out with a lot of pain for customers
I'm not willing to discuss this "feature" in public again]

as you are in SMP anyhow just try to do an insert manually.
Just 1 row.
you seem to require something else <UNDEFINED>
in addition, if your record exists already You may need an 
INSERT OR UPDATE   (if this is possible in postgreSQL)

It could be OK, but I wonder that you insert just 1 value
Eventually, there is something required that is missing?
Check the definition of the generated Caché class,
 

Seems just a typo in INSERT statement.
Target Columns must be in parenthesis.

INSERT INTO HSIPIData.datos_usarios2 (addresses)
       SELECT Addresses FROM HSPI_Data.patient

This article and the related example in OEX may be helful.
Effective use of Collection Indexing and Querying Collections through SQL
 

eg. with SQL:
INSERT INTO... Linked_Table Select ...from Cache_Table
as described in docs

you are mixing up 2 things:

• one is the class that stores data in Caché
• the Class generated as LINKED TABLE that stores its data in postgreSQL.'

so the LINKED describes the structure that is used by generated SQL statements
to work on postgreSQL. Take a look at the storage definition of the class
and see the difference.
But your code in Cache acts as if data were local !!
Either by Objects or by Tables. 
if you issue a %Save() on the class it runs a INSERT OR UPDATE undercover.

a few simple steps.

• if not existing yet create a table on Postgres with the identic columns that you see in Cache or use an existing target
• next use SMP > SQL > Wizzard > Link Table
•  
• It creates a class /table that reflects all columns /properties of the linked table. + storage method that can read, write, delete, insert over ODBC / JDBC on the postgreSQL table.