Hi UffeA
When a subdomain is collapsed under the primary, it will be removed after 7 days if no non-capable, DMARC capable, forwarding data was received, and no XML was uploaded during that time. However if a subdomain is legitimate, but only periodically sends with waiting periods greater than 7 days, then I recommend using the domain overview domain action of “set domains as top level” which will place the domain alongside the primary, and prevent auto-removal.
This is expected behaviour.
I hope this helps.
AM
