When you're in the server and make custom advancements and use /minecraft:reload to reload all advancements, the custom advancements appear in the advancement menu and work like they supposed to do. When achieving a custom advancement, that advancement also gets saved correctly in world/advancements/<uuid>.

But when you restart the server and log back in, only the default advancements you've achieved show up in the advancement menu, while the custom advancements are still in world/advancements/<uuid>.

When you try /minecraft:reload, the custom advancements get reloaded correctly (so they work again in-game), but all the achieved custom advancements in world/advancements/<uuid> get cleared. Which means the achieved custom advancements don't show up as achieved after restarting the server and trying /minecraft:reload

This makes it impossible to play with custom advancements, because the custom advancements will be gone after a server restart.