Next, wrong about that. Warlock Fiend patrons are not deities...full stop. If a DM wants to say you are the warlock of a deity but you still play as Fiend Warlock that's homebrew, which DMs are allowed to do but BG3 as a videogame can't accommodate that.
Asmodeus would like a word with you...
Asmodeus is both a greater deity (as of Descent into Avernus) and can be chosen as a fiendish patron of a warlock pact (as stated in the PHB). An exception, sure, but still possible.
