I already raised a matching issue on the recaptcha module - https://www.drupal.org/project/recaptcha/issues/3481076 🐛 unknown-error code hides actual error message from Google Active
A successful reCaptcha v3 invisible challenge prevents forms from being submitted and falls back to the next challenge type (in our case the v2 version).
By debugging the v2 codes (which were easier to replicate by ticking the box over and over) we found that when the module defaults to the 'unknown-error' code it ends up 'hiding' the actual error returned from Google, which in our case was to do with exceeding the free 10k quota due to a sudden influx of traffic.
The issue applies to all module versions.
As on the other issue, the proposed solution would be to log the actual error message returned from Google when there is no matching 'official' error code returned.
Active
2.0
Code
I've created an MR with a potential solution for this. Basically if there is no known error code then attach the code received from Google to the log message.