Is there any S/MIME signature validation documentation?

We use our custom S/MIME certificates, and it looks like since macOS Sequoia, they do not validate any more in Apple Mail, while they still work in Thunderbird, as well as in iOS. However, when I click "Details", it does not give me any error, but just displays the certificate as "trusted". So I suspect that the certificate does not meet some other unknown requirement. Does someone know of any documentation about how exactly Apple Mail validates S/MIME signatures?