On my SMP with a few datapacks when checking the F3 screen the local difficulty is displayed as "??" and the day counter which should be in the same line is straight up missing.
Mobs hurt a lot and often have a lot of armor and enchanted gear so the actual number has to be high.
It's likely due to one of the datapacks I'm using:
Terralith, Incendium, Nullscape (Only the ones that could affect this)
Can a datapack change local difficulty?
How can I change it in my entire world at once?