Shadow bug with substractive cylinder

Hello. I got a problem. I’m currently making a videogame, and I’m building an underground corridor. The shape of the corridor is simple, a square shape, with substractive semi-cylinder digging in the ceiling. Problem is that when I build lights, I got some strange things on the ceiling of my corridor, like this:

Here is what I wanna get:

So… Does anyone got an idea of what’s going on there, and of how I can fix this?
Thank you.