As we went from 1.20 to 1.21, the Datapack folder name changed.
Perhaps the change in the advancements folder to advancement has caused the newly added custom achievements as Datapack to not be achieved.
Comments 3
Custom advancements being revoked here makes sense, since the developers naturally can't update, manage, and support those on their end. Because of the changes, those advancements become invalid until the individual updates them on their end, and any associated data (like who has earned the advancement) is naturally lost in the process (since to the game that advancement no longer exists or can't be found). Really don't see anything they could possibly do to fix that.
Before updating the world to a new version, you are expected to update all installed data packs to that version as well, since otherwise things might break, like the issue described here. Data pack folders have been renamed, which means the game does not recognize any advancements from it which are written in your save file, so it removes them upon loading the world. This is exactly the same that happens when you uninstall a data pack, and is completely expected.
Please update the description to include steps to reproduce this issue, as well as expected VS observed results.
Please also attach any needed commands, datapacks, resourcepacks, screenshots, videos or worlds needed to help reproduce this issue.