Why are archived members being set back to active?

There is only one scenario that could lead to an archived member being set back to active.

When the Data Wizard runs, you can optionally exclude archived members. This helps ensure they aren’t inadvertently resubscribed in Mailchimp. However, to do this, the Data Wizard relies on the archived status stored within Salesforce—not directly from Mailchimp—because it’s not feasible to perform a bulk query of Mailchimp in real-time during the Data Wizard process.

The challenge arises if you’ve archived members in Mailchimp, but the Primary Sync hasn’t yet run. In that case, Salesforce still sees the member as active, so when the Data Wizard runs, it processes the record and marks it as resubscribed—essentially undoing the archive.