This is a fairly minor side effect of fixing the much more significant bug where bomber symmetry was broken (eg. take the Martian set for example, the red bars, blowing up a climber on one side of them would break all the way through, while on the other side it would not; in this case, the effect on miner tunnels would be similar to what you describe, except based on the direction of the tunnel, not of the lemming).
While I'd love to get this fixed for sure, I'm not entirely sure how to do so without going back to a direction-dependant (rather than symmetrical) bomber, which is a much bigger issue. (Or alternatively, writing code that detects miner tunnels or similar slopes, and applies special, inconsistent physics in such a case.) I remember that around the time I was fixing the bug, I had quite a strong discussion with Simon on #lix, but it's probably buried under millions of years of chatlogs now...