This might be a lang json problem, but a lot of the advancements don't show up correctly

version: 2.6.3

remove the old advancement datapack you have on. The newer repurposed structures changed the key used for advancements so the language files was updated. By using the old advancement datapacks, it breaks as a result.

Let me know if that was indeed the issue

Ah yeah that's the issue. Didn't realise I'd copied them over with the other files.