Hello @SimonVanCauwenberghe  The Klaviyo documentation for Typeform integration specifically says it will create new custom property for additional fields:
Documentation link:
https://help.klaviyo.com/hc/en-us/articles/115000107112#h_01HE0YD60D7PS8BCGY3M0REAJC:~:text=Any%20additional%20form%20questions%20will%20be%20recorded%20as%20custom%20properties%20on%20the%20individual%27s%20profile.%20The%20form%20question%20will%20show%20as%20the%20property%20label%20and%20the%20value%20will%20be%20the%20submitter%27s%20answer.

When we sync a new form response, we'll look for form fields that represent email, first name, last name, organization/company, and phone number. If found, we will automatically sync these fields and set these properties on the individual's profile.

Any additional form questions will be recorded as custom properties on the individual's profile. The form question will show as the property label and the value will be the submitter's answer.

Hi @Maxbuzz 

Yes, I understand that every answer on a question will create a new property. 

However, the hidden field is not a question. It’s existing data from Klaviyo that gets pushed to Typeform to personalize the questions. 

So what’s happening is that I push property ‘Example’ as hidden field to Typeform, and afterwards Klaviyo extracts the property ‘Hidden Field: Example’ and creates a property for it. Although it is not an answer on a question.