If you add the "Delete portal row" script step to a button then that script step will over-ride the default behaviour.
Having "Allow deletion of portal records" unchecked will only prevent the delete key on the keyboard, etc. from deleting the portal row. Script steps over-ride default layout behaviour.
Carl, good point! This is, imho, expected behaviour. I did not mention that my button performs the script step Beep. I should have. Sorry.
I understand now, I was not aware of this bug but now I am!