When you click on the "Statistics" button while the world is laggy, the words "Retrieving statistics..." will appear along with the nice loading animation below it. That's at least happening if you play that world in 24w07a and below. In 24w09a (1.20.5 snapshot, the snapshot that removed the dirt background) and above, that text and that loading animation no longer appear. A "blank blur background" will appear instead. I cannot find any change on the official 24w09a changelog, so this is definitely a bug.

Steps to reproduce

1. Play in a 1.20.4 world
2. Place down an "Always Active" repeat command block with the command /reload
3. Ctrl+pick block the repeat command block
4. Place down as many of these command blocks as you can
5. Open the statistics screen
   → The text "Retrieving statistics" and the loading animation appear
6. Now upgrade your world to 1.21
7. Open the statistics screen

Expected Results

The text "Retrieving statistics" is shown to indicate what's happening

Observed Results

The text and the loading animation don't appear and the screen is completely blank (just showing the blur overlay and nothing else)