I suspect it's because you leave it on standby. Or a solar panel is connected at night. There is always some current flowing away. You should press the button until OFF appears. Remove any solar panel at night. That way I don't have the problem.

I have also noticed this behaviour with my River Max, that the battery ran down in a very short time, e.g. from 35% to 0%, with only a small load connected (4 W). I then fully recharged the unit from 0 to 100% with a gentle charge, i.e. 100 W, then used it normally again down to approx. 5% and then did the same 1 or 2 more times. After that, the calibration of the charge level indicator returned to normal. I found the tip on the Internet. Before that, I had charged the unit a few times in between, e.g. from 60 to 70 %. I suspect that this messed up the charge level display. PeterF