Hi @rfg76,
That is pretty mysterious because iOS 15.1 definitely accepts the X1 root, and, as you saw, you have an A+ on SSLLabs including a simulation of an iOS client.
Is it possible that the iOS client in question is on a network that is interfering with the connection in some way, such as an intercepting proxy or firewall? Is it your own iOS device, or did you get a report from someone else about this compatibility problem? Can you get a screenshot of the specific error message?
Which protocol and port is the client using? Is it some particular app under iOS? Is it HTTPS on port 443 or one of the e-mail services on 587, 25, or 993? (Though all of those also look right to me.)