I think what's confusing here is the speed - bombs are falling slower than you expect - I was able to outrun my own bomb with a dive.
Nah they way it was tested, I was simply waiting to see the the explosion. It was done with two different PC accounts and two different screens, both controlled by me in the same privatised session. Some bombs could be seen by the 2nd account player on the 2nd screen falling right on top of the player but then just disappearing without any effect, other times the bomb had already disappeared. The only thing I can recall doing before testing that was having dropped a lot of bombs already prior to the altitude testing, and that I was right over Fort Zancudo, about a 10 second sprint away from the hangar.
Could it be that there's something (a script) that despawns the bombs purposefully when you're over X altitude at Fort Zancudo and LSIA, when there's another player in there/ near their hangar, as an attempt to prevent people abusing that mechanic? I noticed that both YT videos posted on the previous page didn't have another player in Ft.Zancudo nor in LSIA, but I've not done any testing on it yet to see if there is anything at play.