Why might my multiplus II shut down for 90 seconds?

DIY
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1

My system has been in place for about 20 months and is working nicely. 5kW Multiplus-II, 7kW solar with 3kW on old AC Fronius inverter, and 4kW on Victron MPPT to Fogstar 16kWh battery, in the UK.
But this morning at 6:30am the house power went off for about 1.5 mins before coming back on.
There were no errors reported, no significant loads.
The battery was at 8% (so quite low - the discharge limit is set to (5%), but showed no errors or warnings during the outage. The multiplus and cerbo reported no warnings. The system had delivered 1.5 kW to the microwave at 6:02 and 3kW to the kettle at 06:15 without any battery sag. There are no automatic loads in the building over 100W. No-one switched anything on when the power went off.
There is nothing to see in any of the vrm monitoring graphs at 06:30 except the AC output voltage dropping to 0. The VE.Bus state was ‘Bulk’ and went to ‘Off’ at the time (from 06:29 to 06:31), then ‘Inverting’ for a short period - maybe 30 seconds, before going back to ‘Bulk’. Household load was steady at about 80W for most of an hour before and after (apart from the kettle and microwave loads noted). The MPPT had started up at about 5:20am and was at about 50W (DC-coupled Victron), 150W (AC-coupled Fronius) at the time of the glitch so the battery voltage was rising gently up through 50.9V (from a low at 5am of 50.7V).

So all of this seems rather odd to me.

I presume what happened was a shutdown/reboot of the Multiplus II inverter. Why might it do that without sending any complaints? There is nothing to see in the logs of the cerbo, except it spotting the network disappear (because my router/AP went off with the power).

There is a sudden fall in the AC Input graph at 06:29 from a steadish 247.7V to 237.2V, before going back up to 245.7V at 06:30, as the AC output dropped to 0V, so maybe that’s what caused it? But the Multiplus shouldn’t stop internal inverting if the external grid glitches or drops out?

Any other possibilities? Is there such a thing as forced/automatic firmware updates for the multiplus II. I doubt it - that doesn’t seem very ‘Victron’. It has never switched itself off over the last 20 months except when I did a firmware upgrade and it rebooted.

2

In this scenario of the battery is low, it may not have had enough to provide a surge to make the change over to inverting.

I am not personally used the forgstar bms used (read alot on the forums about them). Did the DCL drop at that point or the efective charge voltage limit?

3

Just checking that here you are describing ACout1? Are all your house loads connected to this protected output?

4

Yes. The whole house is connected on ACout1 (which VRM calls “AC output”)

5

It was already inverting, supplying the small house load (80-130W) from the battery - it didn’t need to change over.

9

Do I understand correctly then that the system was already disconnected from grid and inverting?

The AC voltages are (as on it is available) there but you are not connected and synchronised. (So ignored)

The system just went off then self recovered?
what bms is in your model fo Fogstar- the JK?

As a thought what is the offset on the logs for the 0V?
What is the last known time for the GX reboot? Did it reboot during the incident? If it did that would explain the missing complaints and would point to a DC powercut