This problem deserves its own thread I think - I reported it before as part of another issue, but it’s definitely separate. I have exported just flow 5 of my enormous project:
I put a custom bar number at the beginning of the flow, and as you can see, Dorico now skips back and forth between the original (i.e. starting at 1) and my amended timeline (starting at 558). As far as I’m aware there are no other manual bar number changes (is there a way to see these?) and adding extra manual changes makes no difference.