Hmm, line ball call, but OK! I see there's also a piece on youtube described as a Viola Concerto, although the feedback section there suggests it's actually his bassoon concerto, re-tweaked.
Revisiting this quiz, I'm struck by another curiosity - what is this mysterious Mozart cello concerto? Haydn wrote famous ones, but there's nothing by Mozart...
True! Although the answer opera premiered after The Clemency of Titus, they seem to have been written in overlapping time in 1791, so we've tweaked the clue as you suggested.