Read summary.

How to reproduce:

1. Go to a village and find a Nitwit, trapping it in a box for testing purposes. If there isn't a Nitwit, summon a villager with its "Profession" NBT tag set to 5.
2. Do "/effect give @s bad_omen"
3. Right click on the Nitwit

          →  The Nitwit doesn't sweat.

Possible cause:

Now, I'm no Java scientist, but I think the "sweating" effect overrides the trading GUI during a Raid. And if a villager doesn't display said GUI? No sweat! (Pun intended)