Symptoms: TLS Works From Some Clients But Fails From Others

• The HTTPS website works fine in Chrome and Firefox on desktop but fails on curl, Python requests, and some mobile browsers.
• curl reports: "SSL certificate problem: unable to get local issuer certificate."
• Python requests raises ssl.SSLCertVerificationError: certificate verify failed.
• Desktop browsers show a valid green padlock with no warnings.
• The SSL certificate itself is valid and not expired (checked via browser).
• The issue affects all endpoints on the same server/load balancer.
• An external SSL checker (e.g., SSL Labs) gives a grade of B with "chain issues" noted.
• The problem appeared after a recent certificate renewal.