I would like to replicate your issue but am having trouble doing so.
"You will find that even the portal records that are supposed to be hidden are retrieved."
Portal filtering does not filter the relationship. And scripts, unless you are in the portal, will see ALL related records. If you have filtered a portal so there are no records then you cannot even go to the portal (unless 'allow creation' is turned on in the relationship) in which case you should be able to go to the first row to create a new record.
" that portal filtering omits records from a found set,"
Portal filtering doesn't affect the found set of the parent (the layout you are on). Can you provide us with step-by-step to reproduce what you are trying to do? You may have discovered a bug in the lastest updater but odds are, it is misunderstanding. :^)
Many thanks for your reply! After some experimentation, I found the problem, and also found documentation:
"Go to Related Record
This script step can potentially expose all the related records for the portal, not just the filtered records. This is somewhat unavoidable, akin to how a portal with sorting enabled would not go to the new layout with the records sorted the same way as the portal had been."
Thus, I must have misunderstood how script steps work with filtered records in portals.
If you go to a filtered portal row before you evoke the GTRR, it will go to only the filtered records. ;-)