Datapack advancements not saving - Modded Multiplayer

I’ve made a 1.18.2 data pack focusing solely on advancements. The datapack uses some modded variables such as entities and items as it was created for a modpack server. 
All the advancements work as intended. However, none of them save as a player logs out and back in. They simply disappear. 
Vanilla Minecraft advancements stay and work properly. 
Basically, I have no idea if it’s a code issue,  perhaps a mod is interfering with the saving of it, or I need to enable something server-side. The datapack was placed into a worlds/datapack folder.