How do I fix "This site can't provide a secure connection"?

This error occurs on an Iphone with iOS version 14+ (on version 13.4, everything works without problems):

A little background: There is a site that is located on a subdomain: subdomain.example.com

For the domain, CloudFlare is enabled with a DNS CNAME record with content example.com and named subdomain

In other words, requests from the subdomain go to the main domain and are already processed by the web server there. In the settings, CloudFlare is enabled Tls 1.3 support, free SSL is enabled and the "Flexible" mode is set. On other platforms (Windows, macOS, Linux, Android) everything works without problems. I understand it's about the IOS security policy, which for some reason can't establish a secure connection. Maybe the fact is that SSL is actually issued in the name of Cloudflare, and not on the domain example.com?