Full-keysounded osu!mania maps required every custom hitsounds to play as mapper intended. With this option on, every notes will just play random sounds and screw up the entire song.
Some example maps here:
https://osu.ppy.sh/s/63089
https://osu.ppy.sh/s/92190
https://osu.ppy.sh/s/136986
https://osu.ppy.sh/s/235098
Some example maps here:
https://osu.ppy.sh/s/63089
https://osu.ppy.sh/s/92190
https://osu.ppy.sh/s/136986
https://osu.ppy.sh/s/235098