No, the address does not have to match the plug location on the DBH. The DBH just provides wiring between the cables plugged into it.
If each keypad and the P212S is configured to a different address, then that leaves some sort of wiring problem as the cause.
If the wiring to keypad 2 has a problem on the return wires, then the entire data bus is probably not being terminated properly. Even without proper termination, some devices still might work and others might not.
Since your cable is short, and you'd rather not re-terminate it, I guess I would try the following things:
1. Look closely at the RJ45 to make sure all the wires are inserted all the way in to each pin, and are ordered correctly.
2. Re-crimp the RJ45 that is already there. It might be that it just didn't get crimped with enough force to make a good, solid connection to all the wires.
3. Use a multimeter or cable tester to check for conductivity from one end of the cable to the other, and also for shorts between the conductors.
4. When you spliced the Cat5 to the keypad pigtail, did you get the color coding right? Orange/White and Green/White to the green wire of the pigtail, and White/Orange and White/Green to the pigtail's white wire?