Confirmed that this can be reproduced on other levels too. In particular, you can reproduce it easily on Fun 9 by letting the lemmings turn around, blocking at the end, then with a lemming walking back towards the start, build so that the builder starts under the edge of the entrance (on the side closer to the water).
It seems to be a result of either encountering a wall while falling, or a physics oddity with the narrow block (which is solid only near the middle - this is generally used as a replacement for invisible tall pyramid blocks when converting L3D levels, due to the original shape being ascendable in Loap).
EDIT: Can confirm this does not happen when fallers encounter a wall that's on a block boundary.
EDIT: I've narrowed it down to the "check for wall in front" code reporting a false negative (at least while falling) for solidity mid-block that's absent at the edges, but have not yet figured out why this happens. I can however confirm it seems to happen regardless of height - I've managed to get lemmings stuck at the base of a tree rather than part way up it.