Yes, that appears to be the same issue we ran into. Sorry for the lack of detail - I was mostly concerned with the new api. Here is the html response we received when trying to submit to a form with reCAPTCHA enabled.
It appears there's no support for this, since it says "If you've manually integrated this form, please turn off reCaptcha". If that's the case, it's fine, but I would recommend noting this restriction somewhere in the api documentation. It could help avoid some confusion for developers in the future.
Upgrading to the v3 Api works for us as a solution and I'm glad to hear it should continue to work with reCAPTCHA enabled forms. Again, it would be helpful to note in the docs that reCAPTCHA will be ignored for api submissions (or however the api handles it). When working with APIs it's not ideal to have to make assumptions, since it can come back to bite you – as it did for us
Thanks again for your help!