This is a well-documented phenomenon. I recall reading that bassoon and horn are the two instruments with the most similar timbres in the orchestra (maybe just of the wind instruments), and for this reason bassoons often function as additional horn parts or provided chromatic lines unachievable by natural horn

@@TheGymSavant thanks a lot, very well said!! but it's hilarious how Mozart even mostly writes for bassoons here (and in K. 447 also) like he writes for horns (same intervals and rhytms) 🤭

@@hjo4104 It’s also worth noting that there is speculation this work was not actually written by Mozart. We know that this was actually the last horn concerto written, that it went unfinished before his death, and was finished by one of his students. The material that was finished by him (the first movement in particular) is quite uncharacteristic and unconventional for Mozart (and not in a good way) so it is speculated that the autograph of the first movement and part of the rondo are Mozart “brushing up” someone else’s work, possibly Leutgeb’s?

Further reading if anyone is curious, this is quite thorough: www.historicbrass.org/edocman/hbj-2004/HBSJ_2004_JL01_004_Perl.pdf

@@TheGymSavant I did not know that. I know that the second movement was finished by Süssmayr I think, but this is an interesting info. It could be possible that Leutgeb finished it using Mozart's sketches.