What could cause condensation on a ceiling after painting with a roof sealer?
According to the Technical Data Sheet for Lanco Urethanizer (direct link to PDF), or at least from the way I'm reading it, "Lanco Urethanizer" is meant for external applications where you need to make a surface waterproof.

Therefore if those tenants have used it internally to paint walls &amp; ceilings, they have sealed those services ie Made them waterproof.

If I have the above correct, I believe that would explain why you're seeing condensation on the ceiling (and probably on the walls, especially higher up the walls) ie Moisture in the room can't penetrate the waterproof surfaces, instead it just sits on the surface as condensation.