Missed one check.
This time on the percent read, not the last time read. The background fetch is a different code path and is what always use, so it is much more thoroughly tested.
The attached beta fixes this problem.
But, before you try it, could you do something for me? Could you post or PM me a full log when this happens. There are some things in that log that do not make sense. It isn't the full log, at least some lines at the start have been removed. I would like to look at the full log to see if I can work out what is going on.