OK, surgery was approximately 3 months ago. Buttocks ache started 1 month ago.
Your op report (only for the posterior approach) notes only left sided screw placement and no intraoperative guidance imaging (O-arm or other) but robotic guidance and electrical stimulation which was reported as normal. Since your leg pain is right sided, I am going to assume that left sided screw placement is not the cause.
What can possibly occur in some XLIFs (which are typically left sided approach) is that the spacer cage placement can occasionally push out a small herniation to the opposite side. Either that or a stretch injury to the nerve root can occur. In both scenarios however, typically the discomfort starts within a week from surgery but some delay can occur.
The proper treatment is first to give oral steroids, then if no response, new imaging would be in order.
Dr. Corenman