i think trying a different SIM (if possible) would be the most obvious test for now.
The fact that it still works in the FP2 might reflect that its “tear and wear” results primarily from that device. Maybe the scratches etc. the card has by now “fit” perfectly in the old FP2 SIM slot.
Short update:
After redressing the card it worked for a couple days then failed again.
Last week I went to a Lycamobile shop to buy a new SIM card and called them to transfer my number on it (They don’t send replacement SIM, that’s the procedure )