Is anyone besides me getting a warning that this building is not connected to the colony when building it adjacent to a tunnel? It builds there and works just fine. Is this a bug overlooked by the programming house for over two years, or some artifact introduced by GOG, or my machine being weird, or what?

I have the same issue, so I doubt it is just your machine.

I'm getting the same issue myself, but logically I know that it will be connected when you build it net to a tunnel regardless of the error message. I build it anyway in a place I know will work based on my exprience with other structures, and it works just fine.

Thank you, both of you. You would think after two years that the programming team could have fixed it, but it is what it is. Pretty entertaining game, actually.

Very much later... Just to let anyone else having the same issue know: it's still not fixed.

Glad to know it's just a bugged warning, not an unbuildable structure.

Ignore it.
I find it is better to build subject to drone range not connectivity (especially turrets and construction drone hubs)...... Then connect it afterwards.