In the case where the background image of individual difficulties is
different, querying the beatmap *set*'s metadata as we were will cause
issues.
I haven't added test coverage for this but can if required. Can be
manually tested using https://osu.ppy.sh/beatmapsets/1595773#osu/3377474
(specifically the highest difficulty).
Closes https://github.com/ppy/osu/discussions/16873.
Completely aware that this isn't how it should be done, but would like
to get this out in a hotfix release today. Maybe changes opinions on
https://github.com/ppy/osu/pull/16890 structure?