For circles only use circle programs.Vectors go off round.
Engraving,cutting tool supports usually have a flex in them after a while when using excessive speed or depth. SAFETY FIRST Power OFF! Forceably by hand move in all directions . Sideways then flex top to bottom. Also a slow cutting speed when change of direction reaches a 45 degree angle combination allows the tool to bite in and forceably move circle out of round, slowly correcting itself as forces start to neutralize or combined inbuilt spring forces fights back to perfect alignment. Excessively oversize tools for the cutting motor would also reduce cutter speed to a BITE-IN scenerio Replicating your photo. Trust this covers all bases,
Regards Stephen Fitton (Designer/Builder-Impossible-Machines!)