We are having exactly the same problem with our website since the upgrade to this plugin and the move to reCAPTCHA v3.
In the old version we never saw any spam but with the latest version of the plugin we are getting two or three spam messages every day.
I’m with the OP, either it needs an option to go back to the previous version of reCAPTCHA or the v3 support needs significant improvement.
-Andy.