Time Remaining has shown <1 min for 45 minutes

I'm backing up my music library. Looking at the display of the current file being backed up it is working through the directories in alpha order and is less than half way through. After about an hour the remaining time dropped to "less than 1 minute" and has stayed there for 45 minutes. The backup is clearly progressing, but the estimate is so awful I can't imagine how it's being computed.

I see there are other posts on this. One refers to "ATIHIIOXII-1504", but I can't find this in the KB. Google can't find it either.