I wouldn’t say this is necessairly a bug but I could be wrong. The same square can be seen on the landing lights. For example, the Dash 8. When the props are on and you turn the landing lights on you’ll see the landing light with a clear square. The square is boundary of the light texture. Same thing is going on with the strobes. Because you’re viewing the strobe effect on another aircraft that’s likely why the boundary is viewed as black as opposed to when you view the strobe on your own aircraft its clear.
If that makes sense. If you fire up the Dash 8, turn the engines on and turn on the landing lights, you’ll see what I’m talking about.
TLDR; what you’re seeing is the lighting layer.