My FP3+ had two sons installed. SIM 1 is a Three Mobile on a monthly contract. SIM 2 is a Vodafone PAYG.
I like to keep SIM 1 as my preferred SIM for calls, data and texts but each time I restart my phone SIM 2 becomes the preferred SIM for calls. This problem has only occurred since upgrading to Android 11, I think.
It’s interesting to note that my wife’s FP3 doesn’t have this problem. Her two SIMs are Three Mobile and Giffgaff.