Server side validation Expression (Is it a bug?)

Hi,
Could someone help me to check if this works? Or if its a bug?

When I create a server side Validation and put in a POST variable as Expression it works.

image1402×1016 73.1 KB

But when I put in an expression like {{$_POST.upd_userName != getOldUserName.userName}} its not working.

image1398×1020 68.8 KB

The thing I am trying to do is to only “activate” the validator if new password received from POST
variable is different from old one fetched from DB.

Should it be possible to put in Expression in this field?
If so, could somebody please try and see if it works?

Thank you.