ERR_CERT_AUTHORITY_INVALID on iPhone
Why it happens on iPhone
A configuration profile or MDM-installed root certificate on iOS can be present but not enabled, so the chain validates on other devices and fails only on the phone.
The fix on iPhone
Settings → General → About → Certificate Trust Settings, and enable full trust for the root in question. If no profile exists, the server is missing an intermediate certificate.
Is ERR_CERT_AUTHORITY_INVALID happening on your domain?
An external SSL check builds the chain from scratch with no local trust store shortcuts, so it shows a missing intermediate that your own browser may be silently caching around.
Free, no signup. Runs from outside your network, which is the only way to tell a server fault from a local one.
If that didn't fix it
iPhone has its own failure mode, but ERR_CERT_AUTHORITY_INVALID has a wider set of causes that apply everywhere. The most common one overall is: the server serves only the leaf certificate and omits the intermediate bundle.
All causes and fixes for ERR_CERT_AUTHORITY_INVALID