I’ve had the exact same issue for many months. I have a windows dell PC and same issue.
Quite annoying. I was able to get rid of the lock symbol by going into settings on the Apple TV 4K, turning on require password, then deleting the existing password and pressed done. The lock symbol was gone. But with iOS 13, you must have a password and it will auto generate a password if you delete it.
I have two other older Apple TV’s 3rd gen and music streams just fine to them, just not my Apple TV4K.
I had iOS 13.0 BETA and this was an issue then. I got another Apple TV 4K, had iOS 12 on it and worked fine. Then iOS 13 came out, installed it and the same issue.
Several weeks ago, I called Apple, spoke with a senior tech advisor who sent my issue to engineering. I called again for an update and they just told me engineering is aware of this issue and it will hopefully be resources in the next TViOS 13 update. We wait.