I would have to recommend confirming it on your dev site and reverting registration form customizations and any add-ons that interact with registration in anyway. Nothing changed with the CAPTCHA system itself, but the registration code has changed significantly so I suspect a customization is interfering.
I have questions & answers CAPTCHA verification set. When people register all they see is the "Verification" field. Then when they get an error message that they didn't complete the CAPTCHA they can't find it because they don't see the word "CAPTCHA". Is there a way to add "CAPTCHA" before the word "Verification" so that it's more clear to them?