Two days in a row I've had this problem. I was killed and when I returned to the spot, the only items there were what I was holding/wearing.

The first time I was killed by an Enderman in a mine. When I returned, far below the time limit, the map in my hand was the only item I could find.

The second time I was killed by a witch and when  I returned to the spot all I could find was the armor I was wearing.

I lost enchanted tools, and then I lost the enchanted tools I made to replace them. There was  nothing unusual about either death, and they weren't related. I wasn't near lava and none of my items had the curse of vanishing on them.  The items weren't picked up, I had a nearly full inventory both times and I found none of them. I died close to my spawn spot and made it to the death spot in under a minute both times. The armor I was wearing was still there, so it was under the 5 minute timer. Both times it happened I was playing the mobile version.