Hi regarding the warranty having expired, you can now take the bottom module apart to see if you can better clean it.
There is always a possibility that a fairphoneangel near you has a bottom module they can provide to test that it is actually the bottom module.
For a breakdown of the module
However the flashing/disconnect could be a software issue. Have you tried a Safe Mode start
Regarding the inconsistencies::
I have two FP3+ and they have been inconsistent for two years and I have been using a magnetic adapter for six months on both
Also the notification about Rapid and Slow charging are not always relevant. Occasionally I get a ‘Rapid’ charge from 20% annotated with 1.5 hours to full other times it will say Rapid and 5 hours:
I’ve just this minute tested:
The battery is at 63% and ideally under Rapid charging would take about half an hour yet the screen shot shows 3 hours ???
The Power Management Chips, which may well control the charging rate and screen display, are on the main board.
* Qualcomm PMI632 power management IC
* Qualcomm PM8953 power management IC