Our plugin just the mobile user agent to switch the theme. So i am not sure yet but have a doubt if google sends mobile user agent or not. If it does send user agent it should use the mobile theme.
Since the beginning of August we have had the same problem. Google-mobile-bot opens the desktop theme instead of the mobile one, we tested this various times on google search console. A normal human user sees everything correctly but the mobile-bot presents itself in a way that – maybe – the plugin serves it like a desktop user.
Our plugin changes the theme based on the mobile user agent. So if the google bot uses the mobile user agent then it should show the mobile theme. I think for that google have bot related to mobile.