I too have this same issue. Still not fixed after a whole year.
I’m using eSim with Fraenk (Congstar/Telekom) and a physical SIM with Vodafone.
Please all open a support ticket at Fairphone, otherwise it probably won’t be fixed.
Nothing of the above solutions helped me with the problem. I need to have my second SIM turned off permanently, otherwise I won’t have mobile data.