okay, looking into this, i think i found the cause. some enemy events in the game have some overly cautious event scripting that saves your spot in the background music before a fight and then replays it afterwards, to make sure the music doesn't restart after every fight. (i can't remember how necessary this actually ended up being - it may have only be an error with mp3 files and not ogg files, like what the final soundtrack uses)

at that point in the story the beach doesn't have any music, just ambient ocean sounds, which are on a different sound channel from the music. so i guess the "save BGM" commands before these fights are doing nothing, and when it hits the "replay BGM" command it simply replays the music from the last time the "save BGM" command actually worked... which must have been in the basement dungeon