(skip to my main question on the last paragraph if you want)
thanks for the answers and you all were right, the mons i tested on "every other gen" i mentioned are mons that were transferred from past gens already so they had no problems since it was already on HT, and the solution for it really is to just have the mons be on HT. but then i did some i did some testing myself after, on gen 6 to gen 9:
while i was testing, i found that some pokemon get flagged while some other dont, then i made this test, and i realized that for gen 8+, the bottom 2 row mons of these saves are mons that legends arceus introduced evolution without trading (with linking cord or their held items) and they were legal, while the top 2 row mons are ones not having a change from legends arceus and stayed flagged
so anyway, my main question now is if you noticed gengar (ultra sun) and trevenant (scarlet) that i selected, they were the only ones that werent flagged in their rows and they were still on OT, i couldnt think of an exception or explanation for them, so i was wondering whats the reasoning for them, thanks again