iOS app intermittently fails with “An SSL error has occurred” for some users — AWS backend with only an A record

I have an iOS app where most users can access the app normally, but a small number of users are unable to enter the app or load its content.

For affected users, the iOS logs show errors such as:

An SSL error has occurred, and a secure connection to the server cannot be established.
or:

A TLS error caused the secure connection to fail.
The failure happens during the initial API requests, particularly the guest-login request. Because guest login fails, the app cannot proceed to fetch the home-screen data.

Backend setup
Backend is hosted on AWS

API is served over HTTPS

Domain uses an A record

There is currently no AAAA record

TLS certificate is valid

Most users can successfully connect to the API

The issue appears to be intermittent and affects only some users/devices

Could this be caused by something other than my AWS server itself, such as:

ISP/carrier routing issues

TLS handshake problems

IPv4 routing

DNS resolution

AWS networking

TLS/certificate-chain configuration

iOS URLSession behavior

Network path differences between regions

Some IP reputation/blocking issue

Since I only have an A record and no AAAA record, could IPv6/NAT64 still be relevant to this problem?

What would be the best way to diagnose this systematically?

I am particularly interested in knowing how to determine whether the TLS connection is failing before the request reaches AWS, versus the request reaching AWS and failing somewhere in Nginx/backend.

Any suggestions for tools, tests, or logs that would help isolate this would be appreciated.