Mine went a bit funny last year.
Similar symptoms to the OP's. A reset sorted it out.
There is some anecdotal evidence that long term use with solar charging can upset SmartGauges(?). I'm certainly a long term user of solar, but couldn't confirm if that was the problem.
I also had a problem with my start battery alternator and for a period of time would temporarily connect the engine start battery bank to the domestic bank. The SmartGauge didn't like that either and had to be 'reset'
They're great tools for battery monitoring, but not infallible.