The fog the the Warped Forest biome is rendered bright purple whenever the player is effected with night vision.
To recreate: go to the nether in snapshot 20w06a and find a warped forest. The effect yourself with night vision (I used the command /effect give @s night vision 1000).
As we already have an open ticket for this on the Java tracker, we'll be resolving this ticket as a duplicate of that ticket. Duplicates MC-171302 - search before posting.