There is a German Topic, with the following mentioned and I assume that this a quite annoying bug, probably one of the beta tester can report this to Fairphone:
People using only 1 SIM, recently struggled with mobile data and nothing helped to get it runnning (no APN reset or new entry…). One user found, that when you enter a second SIM you will see that the mobile data preference is set to always ask (as there is not second SIM the phone will surely not ask).
I’m suprised there even is that preference option for mobile data, because although I use dual SIM since many years, I never had this option with any OS. There is the option to set for SMS and Calls, however for mobile data (as only 1 can be used at a time) the only you have to do is enable mobile data on one SIM and when you try to enable on the other, you will be notified that the other is used till now and will be automatically disabled.
Does anyone now, if this (mobile data preference setting) is indeed something in FPOS I never noticed, or if this has been integrated newly or if this probably was somehow integrated by mistake?