I'm not sure even what that could be still. Could possibly just be a hypermobile joint.
It may help to start doing specific glute strengthening work, hip mobility/loosening in general (especially hip flexors... psoas major in particular) and working on thoracic mobility. Those thins are always good, but would help significantly with a hypermobile vertebrae while doing strengthening work for low back
If hte physical therapy helps I would keep doing it though.... and avoid surgery especially for lower back at all costs especially if you're improving.