Was that definitely via Coventry though?
When I saw your message I had, coincidentally, just checked that -- I was fairly sure that I'd already checked it for the weekday service but less sure about the weekends. It turns out that the 21.45 ex-Reading Saturday service was non-stop from Leamington to Birmingham (so the last Reading-Coventry train was the 21.15), but Coventry was indeed served by the last train on Sun-Fri. Apologies for not checking this properly in the first place.
The pattern in December 2019 was that on Mon-Fri the xx.15 (plus or minus a few minutes) from Reading ran via Coventry and the xx.45 (ditto) didn't, but with no 21.45 and the 22.02 behaving as if it was the 22.15; Saturdays had the same basic pattern except that the 20.45, unlike the 21.45, did serve Coventry. On Sundays all trains ran via Coventry -- I don't remember that being the case in earlier timetables, but I don't have copies to check.