where [(My Cert name)] is more like: "Developer ID Application: [company name] ([random chars])"

if I add an "x" to the front or end of the cert name, I get "Could not find cert for 'x[cert name]' in keychain"

and if I add the cert to my login keychain AND system keychain I get "Multiple matching certificates are found for '[cert name]'. Please specify keychain containing unique matching certificate."

It seems to be related to my Developer ID reporting "Missing Private Key" (according to xcode)

This appears to be due to me using an organization account and not a personal account.

Ah yes, it is because of the missing private key due to using an organization account. This should resolve it