I am having the same issue with my 5kW Deye inverter paired with a Volta Stage 1 battery installed back in August 2023, using it as a UPS i.e. no PV power...
It has been over the weeks slowly decreasing its maximum SOC, I noticed it a week ago it was at 95%, several days later its only charging to 94%, now 2 days ago its only charging to 93%. I arranged for the supplier to come do a firmware update in situ but that has not resolved the problem.