UI Layout Or Size Can Seem To Shift Unnecessarily Post-Creation

## Bevy version

Bevy v0.14.2

## What you did

We created a main menu for our game using straight Bevy UI.

## What went wrong

On spawning the main menu UI, it shifts / changes during the frame after creation. The expectation is that it would spawn with the correct layout and size, not having to shift to accommodate or conform to what is wanted, since nothing should've changed from the values passed for initial creation.

Video showcasing what I'm talking about specifically:
https://github.com/user-attachments/assets/4c440f45-f723-4c69-ab23-92378c421262

## Additional information

My best guess as to what this is, is some form of layout calculations being performed in Bevy UI?

We don't think it's any of our code that is causing this; it still occurs after disabling / removing any systems which may modify UI values post-creation. Tho, if this does end up being a fault in the code we wrote, rather than Bevy UI, or even just intended behavior, then sorry for making this issue and wasting time.