Dmarcian and subdomains

If a subdomain, for an active domain, is added automatically to the Summary page, will it then automatically be removed again, if no mail has been seen from it within the last 7 days? I seem to recall that they stayed put, but now I seem to lack at least one, possibly more subdomains.

If I go to the detail viewer, I can’t choose the subdomains, even though I am certain that historical data is available for it. In order to choose the subdomain, it apparently must be present in the Summary.

I am not the only user with access, so the subdomain could have been deleted manually, but no one can recall having done so.

Is this expected behaviour or a bug?