The thing is: empty jetpacks (both electric and fuel-powered) will actually reset themselves to fueled state when you recieve fall damage
I stumbled upon it when i tried to recreate jetpack disappearing bug in Charging Tables addon when trying to shift-click it to charging bench
here's the vids:
electric jet: http://youtu.be/7OsZvuNUm-k
fueled: http://youtu.be/FUlPZhaWR8I
jets made with creative mode, discharged by continious flying , SMP
apparently fueled jet needs more impact than electric, but i might be wrong