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.
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Confirmed 19w03c.
Dropped entities aren't visible, but arrows in the ground are. From what I can tell, entities that spawn after you respawn are visible, but ones that were there before are not.
Dying and respawning again seems to fix the issue.
I think you can hurt mobs even though they're invisible (use subtitles).