Unfortunately, there is no solution at the present time. Smartthings authorizes third parties at the account level, not the location level, so the third-parties get confused about what devices you have. That’s true for Echo, Google assistant, Ifttt, etc.
Smartthings has said they’ve been working on a solution, but they’ve had exactly the same phrase on the website for the last three years, so I wouldn’t hold your breath.
From the official FAQ:
Can I control multiple SmartThings or SmartThings Classic Locations with Google Assistant?
No, not at this time.
So far the only solution is to give each location its own smartthings account and its own Google assistant account. that works for some people, but not for others, it just depends on what you were trying to accomplish.