The bug
When using /datapack disable vanilla, opening the advancement menu will still show your vanilla advancements you have already completed. Even after /reload nothing will change. After rejoining the server it will work however.
MC-121976
/datapack disable doesn't refresh advancements for player
Resolved
Fixed
2
NB: What can be noted is that the /datapack disable
does not save to disk, upon world reload, the datapack is enabled again.