I booked an exit row for someone in my family (20D on A321-neo). I suspect that since she just made SM for the first time in her life, she was given an "upgrade" to C+ to improve her brand loyalty. I did confirm that her profile did not have C+ upgrades requested. Her email did say that she was able "To keep your original seat... visit My Trips" but we couldn't switch her back online even though her original seat was still blocked out.
Anyhow, we didn't bother calling to change back. Fortunately/unfortunately, her flight was delayed almost three hours, so 3 FC passengers switched flights (increasing to 6 FC seats available for upgrades), and she got a first class upgrade on her BOS-SFO flight (second-to-last to clear). Helped make the 3 am landing in SFO palatable.