The advancement background is not using textures provided by the resources.zip (world) resource pack in the world folder.
- Create a new world
- Place the root.json file in the world data/advancements/custom
- Place the resources.zip file in the world folder
- Reopen the world
→ You will see that the advancement uses the modified bedrock texture but is unable to load the background text
- You can load the resource pack as external one to make sure that advancement backgrounds are working and the used files are correct.
- It looks like it is working correctly the first time you open the world directly after starting the game, but upon reopening the world it is not working anymore.