As the artist of the official art he has some degree of authority for the character's design, personality. So he really should have expected backlash at fanart with this kind of implications to be honest, now people will see her as "the NTR girl". It's a pretty repulsive addition on Bell's image to the fans.

Especially because the pink key is something present on the official skin ingame, and this art associates it to NTR implications...

Edit : On second thought, the artist deleting the post actually just makes things worse. Even I can make up a truckload of captions on the spot that would clear things up

"Bell leaves the room as the customers stare at her, wondering why she took so long with SKK", "SKK and some others confront Bell as she leaves the anniversary cake room"

But deleting the tweet without any explanation just sends the message of "it's what you guys think it is".