If you have set the "handling of undefined" settings to 2 then in either case (you set x to a nullstring OR x is undefined) in statements like

if $d(@x@(whatever))
set d=$d(@x@(whatever))

you get a <SYNTAX> error, because nullstring (the value of x) is not a valid name for a local or global variable.

In a terminal session issue the following commands and check each output with mine:

kill         // kill all local variables
write x=""   // the output is 1 (because of your setting)
if $d(@x)    // the output is a <SYNTAX> because "" is not a valid name
set d=$d(@x) // the output is a <SYNATX> because "" not a valid name

If your output is different from the above, than please contact the WRC
My $ZV is IRIS for UNIX (Ubuntu Server LTS for x86-64) 2021.2 (Build 649U) Thu Jan 20 2022 08:49:51 EST

I keep on nagging...
I expected to see the environment of your IF-line: is it in a block structure or not. Which variables are initialized, etc. But you always give me the same expressiveness information: just the IF-Statement.

Nevertheless, I think, I know, why you get the <SYNTAX> error:

- under "default conditions", executing the following lines gives you an <UNDEFINED> error

  kill
  set a=1,b=2   // x is not defined
  if $d(@x@(a,b))

- but the very same lines gives you a <SYNTAX> error if the setting, how undefined variables should be handled is set to 1 or 2 (I suppose, in your case it is set to 2) because x as nullstring is not a valid variable or global name.

To see the setting in question, go to:
ManagementPortal-->SystemAdministration-->AdditionalSettings-->Compatibility and look for the setting "Undefined". I'm pretty sure, there is a 2 (or maybe 1). The default setting is 0.

Hint: the above mentioned setting is a per job setting, you must start a new job to have the new setting in effect

I think you are leaving out some important information: you have a terminal session, but you are calling either a routine (.mac) or a (class) method, since a standard terminal session does not accept multi-lines (as you wrote above).
And when you call a routine or method, it would be nice to see that function/method. By the way, for indirection to work, you need variables with global visibility. So I'm assuming you're mixing some existing variables from your terminal session with variables from your routine/method.