This is gonna be a complicated post, but please bear with me and read carefully.
1/ the functional issue with interceptors staying on place after shooting down UFO is now fixed (or at least I couldn't reproduce anymore)
2/ there is still a visual issue that some interceptors are exactly over the UFO, but some are visibly some distance from it... I don't care as long as it works, but it looks funny. Visual issue can be seen here:
https://youtu.be/3vngZKRhrWg3/ this visual issue (and also the description of the bug in the tracker) reminded me that I also had an issue with skyranger following UFOs which land... the skyranger for some reason didn't start the mission after the UFO landed... and I could reproduce this now also on the newest build
Again I used the same trick with "1 Day" speed, but I have seen it also on much lower speeds (e.g. "5 Min"), there is even a proof somewhere in my LP, but I don't know which episode. Anyway, this video:
https://youtu.be/sE7k7foX98s shows the issue... I am doing the same there 3-times, first on "5 Min" speed, then on "1 Hour" speed and lastly on "1 Day" speed. First two speeds correctly result in a mission... but the third speed causes that the mission never starts.
If useful, the save used in the video is attached here. Uses nightly build from today (version visible in video) and FMP 1.6.2, nothing else.
PS: @warboy trying to look smart, I guess you may need to add another condition for ufo.isLanded() just like you added ufo.isCrashed()... but of course, that's just my wild guess