Wandering traders do not spawn at all in deep dark single biome worlds.

Wandering traders should spawn at the surface in any type of world when the gamerule wandering traders not spawning is turned of.

No wandering trader spawned on my single biome deep dark world after 1.20. (I tested in a new one to but not as much and it seems to presist)

1. Create a single biome world with the biome deep dark.
2. Make sure you are not under ground or a place where wandering traders can't spawn. (In wather or area with extreemly few spawnable places)
3. Wait and check your computer within at most 40 minue intervals by looking if there is a wandering trader in any of the possible spots they can spawn by the normal spawning rules.
4. No wandering trader will appear. I have dfone this fore more than 28 hours and the probability of this hapening decreases with time.