Habe bei MPP Solar zwecks diesem "Problem" angefragt und heute Antwort bekommen:
"Based on your setting this is normal. The inverter will force AC charging if battery volt actually dropped to the level specified in this parameter (marked RED). This indicates battery volt is too low and so if grid is actually available at the time, AC charging will override for 10 min and then stop. This design is exactly to avoid battery getting too discharged that could cause damage."
In diesem Fall alles gut 
MfG