Previously, our organization was using a normal Oauth2 resource without custom auth, and we could use the 're-authenticate' button available in the query editor.
I have attempted to create a custom auth flow with oauth2. When re-authenticating, I get a 'Successfully authed!' notification on the top of my screen, but the queries are still failing authorization check. See the following screenshots for how the resource is setup. Is this not setup correctly?
I was struggling too with the custom auth some days ago and since it is not too well documented it took me a while to configure it correctly. There still seems to be a bug with an unwanted authorization screen that pops up (maybe you don't have that, but we did and still have it).
What I see from your config which is clearly different to mine is the header.
You can click on "Preview" when you run a query and should be able to see on the "API Request" tab the details of your request. There you can verify the correct URL and the correct authorization. If you don't see the correct API token there, you can be sure that something is wrong in your custom auth flow config.