I commented on an older bug about hostile mobs spawning in daylight but this is also happening to me. I also upgraded my world from prerelease 2.
I can confirm that "Erase Cached Data" does fix it.
This issue is happening for me in 1.14-pre3. It is in the desert and it seemed like mostly it was just skeletons. I could watch them spawn in and out. They were not burning and getting close enough would trigger aggro. After watching a bit more I also saw spiders and creepers and even an enderman. It seemed somehow isolated to that specific area and seemed to persist after restarting the game. It's a hardcore game, but presumably that's not a reason for it.
Just to amend this I found another bug report specific to 1.14-pre3 that wasn't well written but pointed out the "Erase Cached Data" option for worlds that had been upgraded (in my case from 1.14-pre2). Using that option completely fixed my issue.
This was a rude awakening when playing hardcore and sleeping underground when it shoved me into a wall and caused me to suffocate to death. Ah well, the hazards of playing hardcore on snapshots!
I saw the exact same issue in the nether in single player. Ghasts and blazes were having no issue shooting me, although I also think that the fire effect as not playing on me when I was hit.
Happened to me too on the official 1.13 release. I was also in Hardcore to start with, but probably just coincidence? I uploaded a YouTube video, but forgot to turn on the stats: https://www.youtube.com/watch?v=BDYd7XO2ziM
It did not happen on my second created game (also Hardcore).
So I've seen this happen a decent number of times. I regularly play hardcore and just use New World to get a random seed. I always delete my old hardcore game when I die, but currently that means telling it once when I die, but that doesn't really delete it, so an additional time selecting it from my game list and then deleting it and confirming the deletion.
If the window between my deleting the old world and the new one is too small I'll often see this bug. Is there really a reason that the delete operation can't just block until it is truly deleted?
There didn't seem to be any crash reports available. There were a couple of logs, although I don't see anything obvious. Let me know if you'd like me to reproduce the bug to get additional info. I'll also include the logs when trying to get into each game after restarting.
2021-11-06-1 is the first time it happened
2021-11-06-2 is my attempt to load it after it restarted
2021-11-06-3 is the second time
2021-11-06-4 is my attempt to load that