When you have the option "Immediate Respawn" turned on in your world and you die some of your stuff cannot be picked up again i encountered this bug after i fell into a ravine. I think its because you unload the chunks that you died in and therefore the items in mid air cant be picked up for some reason.
I couldnt upload the video file since it was too big so here is a YouTube link
https://youtu.be/QkDtMJ2F8Ps
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