I don't have images or footage for all of this, but my best understanding of the sequence of events:
Two Foxtrot interceptors caught up with a Scout UFO and air combat began.
A popup appeared reporting that a ground combat team had reached their objective, offering a choice between abandoning the mission and starting it. I started the mission.
The air combat mission was frozen with the ground combat mission drawn on top of it. (Edit: I think the "Paused" box would toggle whenever I hit the space bar - including while making my save file - but I don't think it actually ran when unpaused. I wasn't watching carefully but my screenshots and my video show the aircraft in the exact same places throughout the ground combat.)
The ground combat mission proceeded as normal, with audio for both playing simultaneously. See Xenonauts - simultaneous air and ground combat 3.mkv.
The ground combat mission was completed, resulting in the game returning to the geosphere with the air combat still in the background. See Xenonauts - simultaneous air and ground combat 4.mkv for this and the next.
The fighters, without any player input, took down the UFO and the air combat ended.
I'm not entirely sure what the non-buggy way would be to resolve these events - or two simultaneous air combats, if that should happen to someone - but I figured it was (a) pretty funny and (b) an issue y'all would want to know about, if y'all don't already.
Edit: I just realized that I also hit "save" during this - I'm not sure what would happen if somebody loaded the save but here it is: 2021 easy casual 5 simultaneous air-and-ground combat.sav
Edit 2: I checked the folder and grabbed the autosaves too:
From eleven minutes before the above save file: Autosave1.sav
From around the same time as the above save file: Autosave2.sav
From shortly before my post-this-resolving save file: Autosave3.sav
Post-this-resolving save file: 2021 easy casual 6 simul resolved.sav