I have a relatively new MAX BR1. When I first started using it, I had one Verizon SIM and was successfully using it in Slot A. I recently got a second AT&T SIM. I inserted it in Slot B.
After using a few days, I started to investigate what was happening and I now see that Slot A is not detecting a card. Both the Verizon and the AT&T work fine in Slot B, but neither is detected in Slot A.
While I assume that the preference setting do not impact this, I do have them set for using both SIMs and no preference.