I don't think this one is a Parsoid issue. {{safesubst:}} is handled during wikitext pre-save transform. I don't think Parsoid even implements PST, it probably just passed through the text unchanged, and it got substituted when we were saving the wikitext.

The only interesting question is how did that end up in the saved wikitext in the first place. There are some "bugs" in the PST that allow it to happen (https://en.wikipedia.org/wiki/User_talk:Sgeo/null-edit-detector), so someone must have done that kind of thing, but I wonder if it was intentional or accidental.

I am going to untag Parsoid here.