For RNG methods that are too complex (typically usage of super long seeds), PKHeX don’t validate it’s legality. An example of not doing it I can think off, would be validating boxes of mons causing general slowdowns for the user. This isn’t uncommon; SWSH Max Raids/Dens could be validated individually too, but not done via PKHeX.
Any simple stat change or gender change would have resulted in a Legend Arceus Mon illegal, you just wouldn’t know it. And you’ve been told about that a few times now, so keep it in mind
I feel like there isn’t a need to keep asking about whether a mon is legal or illegal. Just submit purely legal Pokémon, basically ones that you caught normally with no cheats and extracted and made no changes, and don’t claim that changing stats would be “legal”, and you’re good.
(also it isn’t an issue; it’s just how PKHeX and RNG validation works.)