I just experienced the following while watching Unity ads on my iPhone and while playing a coin room match on the Steam platform...
When the Unity ads auto-reset, as they do sometimes after an ad completes, and WGT kicks you back to the "Credit Reward Videos" screen, the game reboots on the iPhone and also in Steam. The game appears on both the iPhone and the Steam screen on the PC with the 45 second match rejoin timer displayed on both platforms (for the opponent). The opponent must click the rejoin or whatever button appears and you see the game reloading on both platforms. Before it reloads to 100% the error popup box appears displaying "Saved Game No Longer Exists". I saw something about multiple instances of the game appearing which is definitely not allowed...especially since I believe Steam monitors these things.
So, hopefully this will assist in addressing one instance where games can't be restarted/rejoined after a hard shutdown and restart.
So much for trying to multitask and watch Unity ads during game play too...unless this only occurs if using Steam to play the launcher from the PC.