Since this is the top result on Google, let me reorganize the explanation for clarity:

Key Findings:
After troubleshooting, I confirmed the core issue: ​Apple's developer certificates function as a certificate chain, not standalone files. The complete set must be properly installed for trust validation.

Steps to Fix:

1. ​Download Apple's Root Certificates (e.g., G3, G6) from the official list:
   → Apple Certificate Authority Page

   • If signing requires a specific type (e.g., Developer ID -G1/-G2), locate and install the matching certificates from the right-side list.

2. ​Critical Pre-Requisite:

   • Your personal/team certificates ​will not work until Apple’s root and intermediate certificates are installed system-wide.

   • Why "Always Trust" fails? → Missing root certificates break the trust chain, making manual trust settings ineffective.

3. ​Final Verification:

   • After installing the root CA, ​reinstall your certificate.

   • Check the trust status under ​**"Use System Default"—it should now show ​"Trusted"** automatically.

This issue commonly blocks developers, and Apple’s documentation doesn’t emphasize the certificate chain dependency. By clarifying the workflow here, we save others hours of debugging.