Hello,
A pretty similar issue was discussed here (in French): https://forum.fairphone.com/t/le-telephone-seteint-plusieurs-fois-daffilie-signal/
I personally encountered this issue while I was in Spain with my FP3 and 2 French SIM cards, a few weeks ago. The phone may restarted several times per hours. I got the feeling that it was when I was moving in the country (most of the time by bus). WiFi disconnected and disabled.
I did not try removing 1 of the SIM cards.
Never seen this issue in France before of after having visiting Spain.
At this time, my guess was that it may mostly be related to roaming.