Items disappearing when dropped

I died about three steps away from my spawn point but when I respawned, half my items had disappeared.
Then, when I wanted to move a (DOUBLE!) chest, I destroyed it and all the items fell out. I walked exactly ten steps downstairs, dropped a new chest, then went back up again - maybe 20 seconds to find that my (DOUBLE!) chest worth of items had all gone.
There was SO much in there. I'm very unhappy.