Certain event nicknames don't pass legality check

[Munchin2000]

So as far as I know, events can pass the nickname legality check as technically it is possible for them to be nicknamed (albeit extremely unlikely). I noticed however that this doesn't apply to the shiny Zacian/Zamazenta/Eternatus events. Even after changing the event nickname flag from fishy to valid, I still got a legality error. Any idea as to why this happens? Thanks in advance.

[Kaphotics]

You cannot nickname anything with fateful encounter.