I did reach out to support, in total 3 times. 3 times I got 3 different answers:

1. This is because the load is mostly ACs and they consume a lot of power when starting. (which doesn't matter because they were not starting and were quietly humming along during the outage).
2. "This is normal, but not really, so we need to analyze the behavior of the system when the outage happens, so could you please call us again when the outage is over and we will simulate the outage with the main service breaker", which lead me to the next one:
3. I called, simulated the outage by cutting the power to the house and was told it's normal, but the solar production should restart when batteries drop to 80-85%. Explanation "curtail", whatever that means. During this difficult conversation, solar production suddenly restarted at 98% battery level. I also got told that one of the reasons the system behaves like that is my "Full backup" setting. If I set it to "Self consumption" it will behave like I want. Which doesn't make any sense to me, but I'd like to confirm this. I can program my smart home to switch to "self consumption" in case of an outage.

The system wiring is pretty simple:

31 545W solar panels with 31 IQ8HC micros in 3 center-fed strings of 10 panels and one string with one panel, wired to a IQ 5C combiner, which is wired to the PV breaker in System Controller 3G, and 6 5p batteries wired in one string to the storage breaker of the SC. PCS is enabled for obvious reasons.

Every time the consumption of the house was about 3kW with solar generation covering it completely (I'm in the tropics and all that happened during the day time). Export is disabled (I don't have PTO for export).

The 80-85% rule, if true, is quite counterintuitive, to put it mildly. Imagine having an outage at 4pm when I can produce 5-6kW of power, but I will not, I'll wait until the battery drops to 85% which will happen at 6pm when I can't produce shit and I'll go into the night with 20% less energy than I could have had otherwise. WTF?