With a miner, that's to be expected (it's a bit more surprising that a
basher allows this). The reason this couldn't (most of the time) happen under DOS Lemmings, is that the check is "is the lemming currently inside a one-way walls area" - if they're merely standing on it, but not inside it, they'll take one swing, hit terrain the wrong way, and turn around. NeoLemmix still implements similar logic, but one critical difference is that one way wall trigger areas are automatically "cleaned up" - if there's no solid terrain pixel at a given coordinate, any one way wall trigger area on that pixel will be removed.
The most obvious way to notice that is, if you destroy part of a one way wall, then use a builder inside the destroyed area, on DOS, the builder's bridge becomes one-way wall. In NeoLemmix, it does not. This can be thought of as that one-way-wall areas are part of the terrain itself, not objects (this is
completely inaccurate from a technical point of view, but is a very good way of explaining how they seem to work for the end user).