Greg, the issue is with the local time signatures you’ve used from bar 256 in various instruments to produce 3/4 beat grouping within your 6/8 time signature. Unfortunately the presence of local time signatures causes Dorico to get confused about where it should get its bar numbers from for the rehearsal marks (it’s obviously not ideal, because as you can see it’s still able to draw the bar numbers OK!). This is something that we need to resolve.
For the time being, I’d advise against using local time signatures in this way if you need to use bar numbers in rehearsal marks. It’s not as convenient, but using Force Duration instead of local time signatures for the affected bars should resolve the problem.