Hmm if you're using your API key, you can check the call logs for failed requests. Go to Settings > Integrations > API Key. Click on the call logs for details. Other than that I think you'd have to create a Private App, or just try the form yourself and see what errors come up in the console.
Hello @piersg, I had a discussion with the support guys. They said Hubspot is not logging the request coming from submission form API. They suggested the same path which you mentioned. NOTE: I'm using this endpoint *https://api.hsforms.com/submissions/v3/integration/submit/${portalId}/${id}`, .* Tested the form functionality while developing so we fixed console errors. But now it's in production, one of the customers is facing the issue. So for us not possible to catch what caused the error.