Large 3 phase system - UPS function unreliable

Hello fellow Victron enthusiasts.

I wonder if anyone can help me shed some light on an issue where the UPS function is enabled. However, customer experienced is not great with it.

Take for example one day on the 26th April:

1st Grid fault where it rode out the event without issues:

Then three short grid disconnects all lead to AC out dropping as well:

Settings are like this: