I’ve run into the bug a lot of times that I started a dig, then checked that it has started, and I saw on the last tab how many hours are left, so it started. Then later when I looked again, I saw that digging was not running, so I had to start again (losing many digging hours...).
Very very annoying bug!

This has happened to me a couple times.

I've come across that once. At least once that I can say I noticed and couldn't explain. Perhaps more but just attributed it to my own error.

Also come across another bug where the remaining time to dig isn't accurate, having much longer remaining than it should have. Have seen this happen ~10 times. Typically along the lines of: 10 hrs remaining, go away for 8 hrs, come back expecting to see 2 hrs remaining but instead see 6.

Edited 1/17/2021 06:44:23

I found the cause of the bug!
Drunk miners!!!

Joke aside, this could be a sync issue.

Go Help -> Contact -> Report Bug

I haven't reported it b/c there's no "evidence" to list or show in the report, nor do I have any details regarding criteria when it does and doesn't happen, so presumably it'd be difficult to troubleshoot and not likely lead to identification of the issue.