I have been facing the same problem from Google Cloud API Console for over a day now.
I tried to add a new redirect_uri for an old project to use it in my webapp but it gives me "redirect_uri mismatch" error. Old redirect uri still work as they used to.
I even tried creating a fresh project from the start and then registered a new application. This now gives me "invalid_client" error via google oauth.
If the old redirect_uri redirect as they should, then why doesn't the new one? Does something else need to be done when addding a new redirect uri?
This looks like a bug.
See Newly created Oauth Client IDs don't work for what seems like a successful workaround.