1. What happened

Editor will prevent the user from disconnecting a remote connection to device after a connection has been established and Play Mode has been entered.

2. How we can reproduce it using the example you attached

Create blank project, connect to HoloLens with emulation tab, then try to disconnect after cycling play mode. The Disconnect button will be greyed out and unable to be pressed.

Restarting the Emulation window will allow you to select the button and disconnect will work.