I've had this twice in the last 2 days - we have 2 x GivBat 2.6 (both on 3015 fw) and the top one (ie the primary) of the pair of batteries has switched itself off, the other battery sitting there with green status and all green capacity indicators but not visible to the system / portal.
Pressing the on/off button on the top battery has a beep and then powers on with green status and all green charge indicators - and the portal shows both batteries at 95/100% full capacity.
The primary battery seems to switch itself off when it reaches/nears full charge - it's worked fine in the previous couple of months since it's install.
Nothing in the notifications or logs - just that the battery info stops being available between the time periods.
I logged it via email with GivEnergy yesterday but not heard anything back yet...