Map background fails to display in song select after exiting a map in a multi-background mapset with coop mod

[Illuminati-CRAZ]

Describe the bug After exiting a map of a mapset that contains different backgrounds for different difficulties, with the coop mod enabled, the background of the map will fail to appear in song select when using a skin that shows the map background, and will instead show the default triangle background. Exiting before the audio starts appears to also cause the audio to not play in song select. Both problems cease to exist after selecting a different map.

To Reproduce Steps to reproduce the behavior:

1. Use a skin that shows the map background in song select
2. Find a mapset that contains multiple backgrounds, such as the mapset of Help hosted by Pianoluigi, or UnluckyCroco's mapset of UNION (TV Size)
3. Select any one of the difficulties
4. Enable the coop mod
5. Start playing the map
6. Exit out of the map with the escape key
7. Observe that the map background does not show

Expected behavior After exiting, the map background shows in song select and the audio plays normally.

Screenshots

https://user-images.githubusercontent.com/42056854/135356947-bafcedf3-af76-48a0-b0ae-ad30e1301cb8.mp4