Thank you for playing and for your feedback!
On the screenshot there is no bug, though I agree that is not transparent. Upper left tower (encircled) is grade 2 and already has 2 neighbours, so it blocks any nearby building. If you upgrade it 1 more time, it will become grade 3 and unlocks taking 1 more nearby tile.
I was planning to add more tutorial levels, but as it happens on jams, we were running out of time :/