I thought I had the same problem as described. I had a certificate that expired so I generated a new self-signed certificate. I imported the certificate on my iPhone 14 running IOS 18.3.1. I then verified the certificate which allowed me to see the certificate in Settings/General/ VPN & Device Management. I also enabled "ENABLE FULL TRUST FOR ROOT CERTIFICATES" under Settings/General/About/Certificate Trust Settings. When I tried Safari to the site, I still got the warning message. About 7-8 hours later (doing nothing else with the phone for the certificate), I decided to add to this list. Doing one last test, because I don't want to post bad information, I found the certificate is working now. I know there is a start time for the certificate, but it was well past that start time. Maybe there is a bug with importing the certificate but it is working for me. Again, I had to wait 7-8 hours after importing and trusting the certificate. Now to look at 2 more iPhones and try them.