question

@Simon crewe, I immediately suspect that this may be occurring due to "dirty" power right after the mains come back (not uncommon in a power-outage event, it can take a while for the voltage to stabilize after the grid comes back online) that may cause the Multi to initially reject the AC input; this is virtually impossible to troubleshoot without a great deal of additional data, however, such as:

How old is the installation? Has the unit only just recently started doing this, or has it been going on for a while - if the latter, how long? If the former, was anything else in the system changed prior to the first observed occurrence of this behavior?

Is the MultiPlus throwing any error codes while this is occurring? What other components do you have on your system?

What firmware version does the Multi currently have? If you have not recently performed any system updates then, just as a starting point, I recommend that you update to the latest available firmware for your unit -preferably through VictronConnect, since it will automatically determine the latest FW version suitable for your model. While we always recommend that a stable system be left as it is, this system is demonstrably unstable and so the first thing I'd recommend is updating the firmware on this -and all connected/associated Victron equipment- to ensure that all devices are running on the most current and stable firmware versions appropriate. That -in tandem with providing answers to the above questions- will provide us a known and stable base from which to proceed with more advanced troubleshooting if necessary.