For HELP-22621 I added debugging messages to help diagnose and see what was going on. Those debugging messages (made in 4.2.9) should be adapted into the current code as wt_verbose(...WT_VERB_BACKUP, "...") messages so that other issues related to backup can just turn on verbose rather than needing to build a special executable.
I've attached the diff I used but it is incomplete. We should return the offset/length like this does but it needs additional messages for when WT indicates copying the whole file. We also probably don't want the output on each checkpoint, or maybe a more limited form of it.