HA looses track of the state of the google hub when that entity becomes unavailable.
Once the display is not unavailable anymore HA thinks it’s still playing the dashboard that it once was, but actually the google hub is idle showing a screen saver.
What is not fun about that is, you can’t call play media if ha thinks it’s already playing that media. 
Only way to fix the problem is call stop media before anything else can play on that display.