Update Existing Subscribers deletes fields

Hi ??

I have a registration form where I register users to events and join them to interest groups and apply tags for each different event.

I obviously want to accumulate returning user’s data so I use “update existing subscribers” so that a single contact in MC will have all the tags and groups added to it.

I have noticed recently that if a user submits a form, any empty field (or a field I have not included in the form but ARE populated on MC) and being deleted from MC!

for example – on MC I have a field called “Kid’s name” which I populate manually post-registration. If, on the next event, the same user registers (and the submitted form does NOT have such a field for the user to fill out), the Kid’s name suddenly DISAPPEARS from the contact.

I suspect that the “update existing subscribers” simply annuls all previous data (but still – tags and groups DONT get deleted from prior entries – that works fine).

Can u help in a way to NOT delete fields which are not directly updated in a new form submission?

THanks a lot