Rear shimano brake lever problem. Slowly returning. What to do?

I’d start with a bleed. Probably some air in the cable.

Disconnect the lever from the hose and see if it still does it to determine if it’s the master cylinder piston in the lever or something else in the system

Likely to be the olive and barb not quite fitted properly, and/or kink in hose.

I’d try a full bleed, flushing the system with new mineral oil. Somethimes there’s a residue of grease or sealing stuff in the hose.