I have had this problem for a while now (I checked the "Mark messages as read when deleted" checkbox but it doesn't work).
My outlook version:
Microsoft® Outlook® for Microsoft 365 MSO (Version 2207 Build 16.0.15427.20182) 64-bit
It seems that the problem is only linked to gmail, because on another mailbox (one related to where I work), the option works.
Useless context: even if it's minor (at first I just recorded it for the bants), it is quite annoying as I receive a lot of emails from slurm and I don't like having unread messages in my bin (Corbeille is the French word for bin).