I am running into the same issue with my product list. The field in SFDC is already multi-select pick list, what did you update the field type in Marketo to be? I have it currently set to string but it is being overwritten.
A multi-select field will still be overwritten if the lead does not select the same values they had before. It's not additive across form posts and sources, it simply can hold multiple values at the same time.
What you seem to be needing is a history field, which is a field that is only appended to. You don't put it on the form, but you push values to it using the Change Data Value flow step.
Yeah we currently aren't doing anything on the form itself, rather doing a Change Data Value flow step based on the forms location. So there is no work around to have it within the same field then?
A single field that's on the form will always be overwritten by another form post, unless you have blocked updates to the field. If you block, it's a complete block, not a partial block. The only way to preserve old values of a field but allow updates via form post is to create a history field.
I recommend avoiding multi-valued fields and instead using multiple Boolean fields in most cases.
We are not using the field on a form in any way. We have a form on a specific web page, just a general global form. Then on the smart campaign we are triggering the product type to be written which is a multi-valued field in SFDC. I was hoping for a way within the smart campaign to add an option without overwriting any existing values but it sounds like it is not possible without the help of a historic field.
Thanks for your help Sanford.