I’m able to reproduce that on a different device of mine. It looks like the SmartThings app has api.smartthings.com added as a Verified Link which means Android automatically tries to open the app when you are redirected to that URL (it won’t happen if you enter the URL directly).
I haven’t dug into it closely yet, but I suspect this was a recent addition or change to the app. If it was intentional to include the api.smartthings.com domain, they should at least be able to filter out the OAuth paths so it doesn’t break things like this.
I’ll reach out to SmartThings to make sure they are aware of the issue.
Edit: In the meantime, you could go into the App Settings on your phone, find the SmartThings app, and disable the ‘Open by default’ option (it might be named slightly different based on the phone type).