The bug

Due to MC-150020 the composter is now under "Decoration" instead of "Miscellaneous" in the Creative inventory. Apparently the recipe advancement was moved respectively as well:

• 1.15.2: minecraft:recipes/misc/composter
• 20w06a: minecraft:recipes/decorations/composter

However, it appears there is no datafixer for this, causing the following warning message when upgrading old worlds and causing the player to lose the advancement.

Ignored advancement 'minecraft:recipes/misc/composter' in progress file ###.json - it doesn't exist anymore?