Well I don't know if you'd call this a bug but due to the EXTREMELY short range of mines, and the 3 dimensional placement of mines, it is EXTREMELY  UNLIKELY that a mine will detonate.  I have tested this and even with ships DIRECTLY on top on mines they will not go off (due to the 3 dimensional plane).

Yep, I also noticed that the mines rarely work.  I filled a couple of grav. wells with mines to try them out and the only thing that set one off was a lone scout frigate.  Unfortunately, the ship was well past the mine before it decided to explode, lol.  I'm watching the ship run straight over a ton of mines and suddenly one explodes on the other side of the planet from the ship.  It looks like it just needs a bit more range and a more immediate boom.  Hopefully they'll get it sorted soon.  I'm enjoying Entrenchment so far, though, since everything else seems to be running smooth, as far as I can tell.

Yeah the pirates seem to be obliterated by them.  Definitely makes them less of a problem.

And yes, building a mine field and then cancelling production ends up giving you 10x the resources and credits it took to place them.  I think someone else pointed this out in another thread.

TEC proximity mines cost 1000 cr, 350 m, 250 c to place a field of 10.

And with the bug you get a total of 10,000 cr, 3500 m, 2500 c when you abort building them.  Kinda' a big bug if you ask me.

EDIT:  Does this bug have to do with each mine being counted as a separate structure when the whole field is built as itself one structure?  I've noticed that the notification when one mine is completed is "proximity mine field competed" or whatever.  Just for one mine.