Good catch! Thanks for the feedback, I've fixed and updated the demo link.
It's been a while since I looked at this and in doing so I also found another issue: it used to be the case that if you clicked on an empty tile it would still register as being over the tilemap, but a while back (after me posting this demo) this changed so that empty tiles no longer trigger as over. I've updated so that left clicks work again.
Also the tile update was repeating whilst the mouse button was down, even if you were on the same tile - I've adjusted the code so it only triggers once whilst on the same tile.