Hello! I'm having this same issue. Mail client on my iPhone with iOS 15 complains about the old R3 cert being expired. I can reach my email by any other method/client.
How do I change Apache to "send the full chain"?
If I remove the account, and put it back, I can trust the expired certificate one time. But after a cert renewal, I get the error again, and can no longer trust/override. I have to remove the account again, and re-add it.
The domain is only using the new "ISRG Root X1" path, yet somehow my phone still sees and rejects the old certificate in the old path.