Hello
You are running MRP in parallel, which means that each package is called on a different server by means of RFC. In this case, if you set a break-point on the user-exit, the debugger will be called, as it cannot be called on the RFC.
Therefore, the fact that your break-point is not working does not mean that the exit does not work. You must find another way to test if the exit is actually working, for example, saving the processed records on a custom table or checking the MRP lists after the MRP run.
BR
Caetano