Refer Below:
I was wrong Jumper P1 should be flipped if installed on a metal wall or junction box. (See 2.9)
Tamper is correct (2.6)
So at this point i can only assume that the issue is because it is being fed 12v instead of 24v which it is capable of. Thats my quick and dirty analysis of this atm. But judging by what i see here it should have zero issues reading your NeXt.
In its current state though it shouldnt read anything with the cover off. If you wanted it to function with cover off youd have to move the tamper jumper from 2-3 to 1-2