I don't know if this is a bug. After I applied the mirror modifier on a half sphere, the sphere was acting like I didn't hit the apply button but I could edit the mesh from both sides. I thought this wouldn't be a problem because I only needed the sphere to cut another mesh. But when I applied the boolean modifier the new mesh still had the same problem even though it's not symmetrical.
I haven't used any other modifiers when I applied the mirror and I never entered the sculpt mode.