Just something a little bit strange happened…
I’ve upgraded to firmware ver. 2023.06.26 three RUTX09 this morning and one router as lost the SIM1 pin (the SIM2 was kept)
If I put the pin again and reboot the RUTX, the pin was gone again.
I’ve restored the old firmware (2023.05.11) the pin is here again and kept after a reboot.
It is indeed an odd issue. The PIN code is saved in flash memory for persistence purposes and should not disappear after a reboot. I would say that it could potentially be a flash issue, but it seems that everything else is working fine, and the PIN for SIM1 also persists on the older firmware.
Have you tried updating the device via bootloader? Instructions are available here.