The bug
When a background music is played and you go under water or under lava (3 blocks deep or +, not 2 blocks deep), the music stops playing then. This was not the case in 1.12.2.
How to reproduce
Just go to an ocean or a water/lava lake and the music in progress stops.
Linked issues
Comments 3
Duplicate of MC-131360.
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WAI as from now on underwater background music plays. When coming back on land normal music should start playing.
Sample