I think, this mod was done wrong, the additional remote ir sensor does not meet the specs of a BM7000... some diodes are probably missing. It's not so easy to transplant 5500 technology into a 7000.
Check the signal for CMOS/TTL signal compatibility. Probably the signal level is higher than 5V oder below 0V, then the integrated safety diodes in den 80C32 cpu chip go defect.
BM7000 with this problem, often connected to a link system, have a defect 80C32 cpu. The port for the ir input is gone, overvoltage.
Use a 4-5V zener diode for protection...
The BM5500 used a 80C31 CPU for the controller, it's internally different to the 80C32 of the BM7000.