“Unavailable” Advanced Security Message

my iPhone 15 iOS 18.0.1 Advanced Security Message is turned on and working

one contact using other than a iPhone (msgs go through green) has the Advanced Security message warning unavailable appear

The warning shows up when you click on the thread to which belongs to the said contact in the text app

When in the app touch the arrow beside contact name and scroll to the bottom where the Advanced Security Message appears the contact name is listed with the warning symbol and the word unavailable

no other contacts in my phone (totalling 1058) have this warning

What does this mean when you have a warning unavailable advanced security message?