I have recently added Rudderstack API also on my Hubspot form submit button on the website.
However, in 1% of the cases, I am seeing that the contacts are only being sent to Rudderstack API and not being sent to HubSpot. This a regular HubSpot form so I assume it should always send the contacts to HubSpot.
Check your conditional logic If you have any conditional logic or rules that trigger when a form is submitted and then determines additional behaviour such as sending events to Rudderstack or HubSpot, make sure these are correctly configured and wouldn’t be silently preventing certain submissions from being sent to HubSpot.