I know sometimes the backpack can drop through the foundation, I saw it in a cave in the past two weeks.

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Also died at launch after falling off the building and the pack ended up between the top layer of concrete and some layer just below that. Weird stuff.

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So I suspect a lot of the "backpacks not dropping" is that they are dropping through something and are not visible.

I've also had players tell me they die and aren't finding their backpacks. Chances are they did fall, but as Nocturnalverse suggests it may be simply not visible.

Have been having this problem but only with sleepers. We are on a PVE server with a pvp day last day before wipe so most people's backpacks should be full of items for those who do not participate. When drop on death is enabled for pvp day it appears that sleepers do not drop their backpacks on death. It has been tested and confirmed on our server with staff involvement. 

yes im experiencing samt thing on my server where players complaining backpacks are not there when they die and go back to find them