Yep, that needs fixing (and I see you already fixed it in your experimental version).
One even weirder thing with this, which I predicted upon seeing the report and confirmed as true - try this:
1. Set up glitch as described in your post
2. Place another blocker somewhere
3. Get rid of this blocker in a way that doesn't trigger the glitch (any method should work, including releasing him by digging under him)
4. The glitchy blocker field is now gone!
Nepster probably already knows why this works, but to everyone else: It's because whenever a blocker is removed, the entire blocker trigger map is erased and rebuilt from scratch. The original glitch here arises from the erase and rebuild not being triggered in those situations.
I'd also think (though I didn't test this), in the case of using a teleporter, the glitchy blocker area should free up once the lemming finishes teleporting (and, if applicable, exploding).