This issue (I guess) only happens in rare ocassions .
what I expected to happen :
Every background music must be played after another one ends .
What actaully happens :
(In rare occasions ) They stacked together .
Steps to reproduce :
1. I am not sure how to reproduce , but if this is an issue then the only way to repreoduce is :
a. check the code
b. Play a lot of minecraft with music on and try to listen to the background music .

This is happpening extremely rare , I have met this issue for once only , ( don't really have an idea to record it at that time )

More details : If it helps :
This is happening right after I started a new creative map . Around the first music being played for half , another music played and mixed with it .
(probably irrelevent ) I gived myself a command block , built 10 blocks of wool and destroyed them , builded a platform out of stained glass , and 1 lever . Then it happens .

(Personal opinion : probably difficult to fixed and reproduce and ever tell that it exist . )