I just had a look at the K2 user plugin to understand what's going on. It seems they changed things a little. They do have their own ReCaptcha keys and, if you enable ReCaptcha in their parameters, they will add that captcha to their own registration form and Joomla's built-in registration forms. For some annoying and breaking way, they don't just use the captcha system built-in to Joomla.. because duplicating things is always better, apparently.
Once ReCaptcha and the K2 User Profile system are enabled, they pass every registration through their own captcha implementation, which is why it's breaking.
With that said, it seems like there's a pretty straightforward work around with a line of code that I'd like you to test. Please edit the /components/com_jfbconnect/controllers/login.php file. Around line 109, you should see:
// Check if no mapping, and Automatic Registration is set. If so, auto-create the new user.
Directly above that line, please add:
$app->setUserState('socialConnectData', true);
Test that and let us know how it goes. If it works, we can investigate implementing it in the next release.
Thanks,
Alex