Interesting! No idea why that’s happening
Pretty sure this is intended behaviour as per slicedlime's (Minecraft employee) video
Check if there is only 1 file inside the “advancements” folder inside your world folder.
If you are on Windows press Win + R and paste:
%appdata%\.minecraft\saves
This should open File Explorer.
Search and open the folder with your world name.
Then open a folder named “advancements”
If there is MORE THAN ONE file that looks like this: 61699b2d-0c6a-475f-a0f9-65808d0be2d9.json (random characters), that means the world has been launched with multiple accounts. This is an issue because Minecraft does not merge advancements with multiple accounts.
Can confirm in 25w35a!
I can confirm on 25w32a!
I’ve had this issue when playing on Mac too. A workaround I found is pressing the title bar and resuming the game.