I have experienced this occurrence three times today, I was in a mineshaft and I turned the corner, my character phased through the blocks below him and died. The message for the death was "fell from a high place" even though my Y coordinate hadn't decreased at all. I was able to recover my items. The other two occurrences happened about 20 minutes ago. I had built a nether portal and I was looking for a nether fortress when out of nowhere My character starts taking damage out of nowhere, luckily I was able to quickly move out of whatever was killing me (please note this world is currently on peaceful). Due to this near death experience in the worst dimension I could perish, I decided to go back to the over world, I bridged over to my portal and out of nowhere my character spontaneously combusts and dies instantly, I figured I had died from lava but I was wondering why it was so fast but I looked at the death message and it claimed I had fallen from a high place, I went back to the nether and all of my items were gone and there was no lava, I even checked with a recovery compass and I had died right where I was standing not below