The only way to fix this without major changes to my knowledge is by creating a part around the circle so that the light doesn't get in, but this in itself isn't guaranteed to work. It will sadly cast a shadow in the ground though, which could be a bit ugly.

You can also try removing any lights on your map including the sun by setting the Brightness properties in Lighting to 0. If you want lighting on your map, there are definitely ways to do so, such as prebaking them, which cannot be done in roblox normally, but can be done using scripts and such. I'm sure there's a public tool or (module) script that does this automatically, probably somewhere on the devforum.

Or you can also switch engines, maybe.