Unfortunately, this is currently a macOS Sonoma bug that will only affect you if you have chosen to use one of the following wallpapers: 

• Dynamic Wallpapers: These will revert to the correct wallpaper but might not revert to the correct light/dark/time of day version.
• Landscape / Cityscape / Earth / Underwater / Sonoma: Will always revert to “Sonoma Horizon”.
• Colours: Will always revert to blue.

Please note that the “Pictures” category and any custom pictures you choose to use as your wallpaper are unaffected.

We have reported this bug to Apple and hope that a fix can be put into place in the future.

Are there any workarounds?

(1) Select an unaffected wallpaper

The best work around we have for this is to ask you to choose a wallpaper that is not one of the affected wallpapers, this way Portal should always be able to revert correctly.

You can select your preferred wallpaper in “System Settings” > “Wallpaper”.

(2) Turn off Portal’s “Sync Wallpaper” feature

You can turn off Portal's "Sync Wallpaper" setting. This setting controls whether or not Portal changes your desktop image when starting playback, with it turned off motion visuals will continue to play on the desktop but with the following limitations:

• the macOS menu bar will not take on the colour of the current Portal scene,
• Portal backgrounds will not show in Mission Control or on the Lock Screen, and 
• window tinting will not take on the shading of the current Portal scene.

To turn this setting off, open the Library in fullscreen, select “Settings” in the macOS menu bar > "Desktop" > toggle off "Sync wallpaper".

(3) Turn off desktop motion visuals

You may choose to disable Desktop visuals entirely and instead choose to enjoy the visuals only within the Portal window. 

Turn off "Motion Visuals", open a portal from within the Library and click “Set Surroundings”. Then click anywhere on the in-window visuals to hide the interface.