Unusual set of circumstances, several suspected bugs in one

Issue 1: 5 blocks (placed with collision) had lag-vanished from under me

Issue 2: After a fall with feather falling 3 from 15 blocks, I had died from fall damage (should not take that small of a fall)

Issue 3: I died from a single fall and the totem in my offhand was used up after the death screen came up, rendering the totem useless

 

This was an immediate death from a 15 block fall in the nether, onto solid ground (not lava) with feather falling 3 and a totem in offhand.