Here is a review of the configurations that I have tried.
I assume you have seen my sequence of cases on this problem (3E6-251FD31E-0057, 0D1-25584633-001E, 257-25718918-004B), as they have some detail about configurations that I have tried previously.
For one of the tests using Android 9, Gmail Exchange worked when the device was on WiFi (IPv4) but failed when it was on Cellular data (IPv6). So the problem at least started by that version of Android.
Today, we tried it on a Samsung S10-5G running Android 10. It configured OK, displayed messages once, and then stopped displaying messages. Phone was on internal WiFi the whole time, so IPv6 is not the whole problem. Since the symptoms today were different than I have seen before, maybe there are multiple problems.
My old Samsung 6 phone with Android 7 is working fine with the Gmail Exchange client.
Thank you for your efforts on this!