Incase this will help anyone else, here is the response from Hubspot support:
From what I understand, you are trying to connect some non-HubSpot forms, but there are issues with submissions not going through when the v3 reCaptcha is enabled. I was reviewing our documentation and similar cases related to this topic, and I found information that states that our v3 endpoints in the forms API do not accept submissions when reCaptcha is enabled. We suggest turning off this setting to receive submissions via the API for this form.