not working as on cave level (layer1) not overground (layer0)
http://ringofbrodgar.com/wiki/Mining - scroll down to "Quoting Phaen" look at image
cave entrance works as a "switch to TP" from overground (layer0) to the first cave level (layer1) - where the unmineable tiles are
as you can dig in front of cave entrances at overground it might explain why it's mineable when mapgenbugged to cave level (layer1)
(wouldn't make sense to mark tiles unmineable where you can't mine (overground) anyways i guess)