When using a sub domain in M365 of a domain that already exists, it inherits the password expiration policy from the parent domain and the value for the subdomain will be "not set". It is not possible to set the password expiration for a subdomain, you will receive errors when trying to set this through powershell. I believe the ISPM password expiration policy check needs to be updated to ignore subdomains of parent domains that already exist in the tenant.
