how does the fx unit map ? i know you have checked the mapping but it isn;t clear from your post whether you are also checking the mapping is intact when you experience the problem. when you get the zeroed controller, is it still saying it’s mapped to cc71 ?
in the interests of troubleshooting, have you tried copying the entire part over, not just the midi track, then switch between two identical parts to see if the same thing happens ?
can you test with a filterable midi monitor to see what messages are getting output when you switch parts ?