It may have been a Sky Marshall who took your seat. They are allowed to take any seat they want and the flight attendant can not tell you why. I have had it done to me with them knowing I was a 1K MM flyer. I would complain to 1K Voice since you had a medical issue.

When I was returning from LHR to US after hip surgery I was in the exact same seat as you and the Sky Marshall tried to take it away. UA staff in London was very nice and exaplained the medical issue to the Marshall who moved.

Of course, you are not supposed to know who the Sky Marshalls are, but they are often easy to spot.