After a plane has landed on a runway that is exactly the minimum length for it, the last tile of the runway does not get cleared of reservation. Looks like a possible rounding error to me.
Here's a small save where the issue is visible. (https://files.huitsi.net/tmp/runway-reservations.sve) It's also happening on I had to create workarounds my airports in Aidenmire and Norburn on B-B, since Boeing 747-400 jams 3500 m runways.
This report by zook2 (https://forum.simutrans.com/index.php/topic,20798.msg194968.html) may or may not be of the same issue – the save link has expired so I can't check.
I've recently had a reservation problem a while after lengthening a runway by two or three tiles. I don't remember whether the new, added tiles stayed reserved or the old ones, but IIRC I had recently used the runway tool at that airport to add a few new tiles to the end of the runway, i.e. I did not drag along the entire length of the existing runway. It seems the game did not recognize the new runway as one continous piece, or perhaps an existing reservation was only for the old, shorter runway and was not updated after I added the tiles.
(I hope that was comprehensible)
Okay, I believe that's a separate bug then, though I have suffered from it as well. This one will happen regardless of how old the runway is – the one in my save was built before the planes were bought.
Thank you for your report. I believe that I have fixed this; I should be grateful if you could re-test with the next nightly build.
Looks to be fixed, thanks.