Some App Instances cannot be focused
So, I don't know if this will work always, but I found one workaround:


Switch to the non-showing instance of the app.
Switch to another instance of the app that shows.
Put this app in full screen
Switch to the non-showing instance of the app -> now it worked for me.


I am not sure if 1-3 have to happen on the same monitor, I will keep trying this out in the next days...