this is not a bug, there have been changes to the file structure in 1.16.2 pre1. please refer to changelog and edit your files accordingly.
Bug appears to be fixed in 1.16.2 pre-1: more testing needed, will do soon.
Launcher log attached: I loaded a fresh world with the datapack, save and quit, and loaded it again. also confirmed that the purple biome became yellow again when I reloaded.
Edit: Datapack attached: meant to attach it previously. on my way to a funeral but I'll attach the launcher log tonight.
Can confirm this error in 20w28a and 20w29a, as well as with sliced_lime's example code, both tweaked and left unchanged.
confirmed for 18w16a
I'm also having this issue with a custom dimension, no fix for me yet. I think most of us also have location based avancements?