I’ve had my FP4 for 2 months with no problems. I live in Spain and have a brand new physical Orange sim and a brand new Vodafone esim.
On Friday I arrived in Poland for the weekend and after turning off airplane mode the phone started to reboot constantly. The only to avoid it was to not enter the pin for one of the sims. I’ve done various experiments over the weekend and the conclusion is that as soon as I enable two sims it starts to crash and reboot. Either sim enabled by itself is fine.
Haven’t applied any updates recently, including android 12.
On the way back to Spain now so won’t be able to test much for a while. Hoping the same doesn’t happen in the UK next month! It was fine in Sweden, Switzerland and Germany last week.
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Turned both sims on 20 minutes ago at the airport and rebooted on purpose. No problems so far. Maddening! 
…
But now it rebooted, when writing a WhatsApp message on the bus to the plane. Probably due to changing mobile network cell because I finally moved. It was fine while stationary in the airport bar. After disabling the physical sim again it’s stable again.
Conclusion - negotiating connection to a cell is the issue. But only when both sims are enabled and I’m in Poland (Warsaw).
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Same thing happened to me last year on several occasions when visiting Warsaw, when trying to enable the eSIM from KPN NL. This was one of the deciding factors for returning the phone.