What's the wall thickness, nozzle diameter, and extrusion width?
Don't think this is related to your issue, but I battled a ring in a circular part. The part started off with a bunch of legs and several inches up the part they all join. At that bridge point all the legs pulled in creating a ridge in the ID of the part.
Seemed to be that the bridging between legs was contracting and closing the gaps between the legs slightly drawing them all in.
To prevent this from happening I added an external structure that connected to the legs at a very small point right before the gaps bridged. This held the legs in position and resulted in the smooth inside diameter I needed. I then use flush cutting wire cutters to remove the halo when the build is complete.
As I said, this doesn't seem to be what's going on with your part, but in case it might help someone else.