After the player dies and respawns, all passive and hostile mobs are invisible. This includes skeletons, spiders, creepers, cows, and sheep as far as I know.

Hostile mobs can still attack, but I was unable (seemingly) to hit them, although it may have just been that they were invisible.

This may be the same as bug MC-139122, but I was not playing with another player, so cannot speak to that aspect, and I did not notice that any items had disappeared as in the comments for that bug.