In SW/SH, since you can now nickname Pokemon from others trainers via the namerater if they don't have a nickname, and this includes NPC trades. (So the alolan npc trades from lets go and the island of armor trades)
So I renamed an alolan raichu from a lets go npc just fine in game, but when I try to rename in the editor, its marked as illegal. The same thing happens when I try to rename an island of armor trade pokemon.
Here's the notif I get when trying to rename an NPC trade pokemon in editor:
And here's the raichu ingame, which i was able to rename just by going to the name rater: