I deepened all mines to their deepest possible (average depth 10,000), then built another mine. When I try to deepen that mine by starting the Deeper mining project again (for the 10,001st time), the project shows up as started but will no longer progress; it is stuck at 0%. The project remains stuck even if the average depth falls significantly below 10k, and remains stuck after a save and load.

Dang that's a very unfortunate one.  I fixed it internally.  Thank you.