I have also been looking for answers regarding the soc 100 -> 90 drop and have the following feedback to add.
My setup: Deye 5kw, CFE 5100s, no pv's
My installation is one week old, the first 4 days my soc dropped from 100 to 90, three cycles per day. See the charts that I collected from the Deye iCloud dashboard.
The past 3 days the behavior has changed from 100 to 95 in short intervals. This was without any firmware updates.
Im starting to think this is the normal maintenance behavior of the CFE bms. Everything else is working exactly as expected without any issues or errors.
Also for the guys struggling with the Battery wifi, this vid helped alot.