Thank you for reaching out to Constant Contact API Developer Support, and for your patience. My team is here to assist outside software developers with questions about building into Constant Contact's API.
It looks like we've already followed up with you directly via email, but here is the information provided for visibility on the forum:
In this case, it’s most likely that a PUT call was sent to update the contact(s) without including an array of their list memberships in the request body, which would result in removal from all lists.
Resource for contact statuses:
https://v2.developer.constantcontact.com/docs/contacts-api/contacts-index.html#contact_status
The details about not including that array are here:
https://v2.developer.constantcontact.com/docs/contacts-api/contacts-resource.html?method=PUT
You have to expand the 'lists' property in the Structure section
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