I use Captcha - reCAPTCHA v3 by SharkyKZ ( maybe the most used captcha plugin for J6 ). It works perfectly with all my submit fields, AcyMailing, Guestbook, and Event Booking single registration form but says "Invalid Captcha" when trying to register to an event using Group Registration.
It was working fine before 04 sept update, or the one before, because I heavily tested it.
Due to the nature of group registration, not all captcha plugin works with group registration (group registration uses ajax for registration steps). Since you are using Joomla 6, I suggest you to switch to use POW captcha comes with Joomla out of the box. That will work well
