While investigating the nether depending on the biome you are in at the moment the background changes. But while standing in the Warped Forest, the background doesn't change accordingly.
MC-171038
Background color of the biome
Resolved
Duplicate
0
Gustavo Bautista
(Unassigned)
Unconfirmed
(Unassigned)
20w06a
Retrieved
I think if you press F3 you'll see that the colour of the atmosphere changes when the biome does even if it looks like you aren't in that biome.