@bruceagreenberg is basically right. Getting 5.1 support basically means a major upgrade in the audio player is needed. And then, work on top of that to make 5.1 work.
That major upgrade is already being rolled out on Android (without 5.1 support AFAIK) after a lot of work and testing. The problem is this major upgrade doesn’t only affect this bug, affects many things. Which means it might fix this bug, and introduce other new bugs that affect many more people.
The iOS/tvOS team is waiting for them to confirm everything is stable, and then we will start doing the same transition, where we will probably find bugs exclusive to our platforms. After that, at some point, we will tackle this issue. But as @bruceagreenberg said we don’t have enough resources and we have another ton of work to do in other areas of the app.
So I can promise you we are keeping an eye on this, I just can’t give you a date when it will be fixed.
We’re really sorry about this problem. Thanks for your patience.