I have the contact form in a modular page, and when I configure the .md file with the Grav website instructions, it show “Captcha not valid”.
I have configured the site and secret keys in the form plugin.
Finally I have chosen to use version 2 of Google’s captcha, with checkbox, and it is working correctly.
It seems that versions 3 and 2 - invisible, don’t work well with the form plugin.
Actually, there are more problems with reCapatcha.
I’ve created a bug report but haven’t received any answer from developers.
You could upvote it, maybe they will take a look at it.