Well you are using re captcha v2.
A lot of people say it is a bug from Google
This can happen if user does not select all the correct images and repeats forever till they do.
Read up at:https://groups.google.com/g/recaptcha/c/pc6NjsBc0Sk?pli=1
Make sure you read all of it.
Or google "problems with recaptcha v2 can't select images"
to see more.
Some have weird resolutions.
For example your user, I suspect he could not answer the challenge correctly on quick reply but when they switched to full reply they could because it was a different challange.
Maybe you need to go back to stock captcha and use ImageMagick
Personally I hate selecting the check box and getting the challenge , because sometimes I can't complete it and I am talking non UBB sites as well.