In MC-268812, most backgrounds that previously didn't had the dirt background were reverted to not having it in the Programmer art resource pack. However, one screen, the one that displays "Loading terrain", was changed to having the blurred background despite previously having the dirt background. It's completely inconsistent with the dirt backgrounds of everything else while loading a world.
Edit: The 'Saving world' screen also shows the blurred background instead of the dirt background.
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Okay, I'll add a video of this in Vanilla.

Thank you for your report!
After consideration, the issue is being closed as Won't Fix.
Please note that this is not the same as Working as Intended, as this bug report correctly describes behavior in the game that might not be the intended or desirable behavior, but it will not be fixed right now. Sometimes, this is because the issue reported is minor and/or impossible to change without large architectural changes to the code base.
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Your filename indicates that your game is modded.
While I was able to confirm this in an unmodified version of Minecraft, please note that we do not accept issues in modified environments. This will usually invalidate your ticket, so for future reports make sure to only include footage from vanilla instances of the game 🙂