That's just how backpacks work in DayZ unfortunately. A backpack will keep its contents saved the whole time the server is up, however once it has restarted the pack will be empty (unless its one you are wearing of course).

Tents/Stashes or vehicles are how you keep things between restarts that aren't on your person. Hope you didn't lose to much we all learned the hard way at some point.

Also remember if you have two guns always take your backpack gun OUT to switch weapons, if you try and add a weapon to an already full backpack the game will delete it. Oh an when adding items to a tent toolbelt items will count as a weapon, use the counters on the inventory to see how much room you have left.