I have found an error with how PKHEX filters nicknames, the filter that PKHEX uses to determine nickname legality is different from the filter that Pokemon Sword, Shield, Brilliant Diamond, and Shining Pearl use resulting in some legal nicknames being flagged as illegal. The example I found is the nickname "bitchass hoe". If I catch a Pokemon in Pokemon Sword or Brilliant Diamond and try to name it this the game will allow me to name it that but PKHEX says that the nickname is illegal. If you don't believe me you can try yourself in either of these two games and you'll be allowed to name the Pokemon this. This is just a small bug but it would still make sense to make the filter for nicknames work properly.
029_-_bitchass_hoe_-_404B0D274D6E.pk8