Concrete below U-shape connector sometimes stay after connector-deletion

I've wondered for a long time where these concrete mini-tiles came from.

Now I got to see one in action:

That is below where the connector was.

I tried to force this to happen on a similar setup nearby, but it didn't happen then. So not sure what exactly makes this concrete mini-tile stay behind after connector-deletion.

Was able to delete the concrete mini-tile on the first picture by recreating the connector and then delete it.

This should be resolved as of v0.4.10