I had a similar experience a few weeks back ... on a R/T EWR-LAX trip. Thru the UA website months ago, I had opted for R/T E+ seating for both my wife and me ... window and middle in first Coach row ... on separate tickets. Upon checking the outbound flight info very late in the evening prior to the morning departure, I discovered I had been moved rearward to a middle seat (E-) while my wife's seat was unchanged.

My call to CS (Philippines) was answered promptly (after midnight) but it took one agent and one stateside supervisor 45 minutes to confirm that (1) my seat had indeed been changed and (2) fortunately the seat next to my wife was empty so they gave it back to me. No reason for the change was offered. No further problems were encountered.

So, unlike the OP's story, my reseating was real ... both on the UA website and in the customer service records.