Strat bombers disappeared on the airfield

I had a location where there were 4 strat bombers (lvl2) on the airfield. 3 fighters (lvl2) were returning to that same airfield too. I decided to upgrade the fighters and clicked the upgrade button while they were still enroute.

Coming back later, near the end of the expected upgrade, I find to my surprise, that nothing seems to be upgrading - no separated stack of fighters upgrading. Just a full stack of 7 planes.

I then split off the fighters and give them a patrol order a bit away so they would go there away from the stack. Now they show up as upgrading and I have 2 stacks.

All good? No? The upgrade finishes, the fighters take off - correctly upgraded (lvl 3) - and the strat bombers simply disappear!

The bombers are nowhere to be found, no newspaper report about losses and no damage to any airfields.