When saving a world either in creative or survival, your choice of inventory GUI layout dosen’t save. Meaning when you reopen the world it goes back to it’s default state. (The Crafting Book for Survival and Creative in Creative.) This means if you use another layout other then the default, you’ll always have to re-pick your layout every time you load the world.
When you pick a GUI interface it’s to save with the world and when the world is reopened it is to be the same type as when you last opened it.
1. Create/open a creative or survival world.
2. Open the inventory and select a different GUI type from the default one.
3. Close and save the world.
4. Re-open the world and open the inventory once again.
5. Observe that your layout is back to the default and not the one you previously used.
If the steps are too confusing I can make a video showcasing this, it’s just that it would be a very long video and one i’de have to upload to YouTube for. If this is necessary just ask and I will provide, but for now I wont attach one.