To be fair it should say unknown rather than insecure if the secondary can't be accessed and it can't confirm that it is secure and you should assume an attacke

To be fair it should say unknown rather than insecure if the secondary can't be accessed and it can't confirm that it is secure and you should assume an attacker able to perform a Man in the Middle attack on you could disrupt your traffic sufficiently to force the fallback to to the secondary server. Although does the validity of the incoming mail servers certificates actually matter? In an ideal world other hosts would never send to improperly signed hosts or without TLS but that clearly is not the case so I'm not sure what current practical value having proper certificates actually has. In the longer term by pushing people to adopt proper certs it may become practical for people to turn off sending to servers without proper certificates so this may be better seen as part of a long term effort.