So, here's what I see as I test things.
In /Library/WebServer/Documents...
If you have no "index" files in the directory you get "You don't have permission to access...."
If you have one file called index.html.en you get that file loaded.
If you have one file called index.html you get that file loaded.
If you have two files called index.html.en and index.html you get index.html file loaded.
I assume I was dealing with the browser cache and/or the Apache cache once I started playing around.
Here's the previously unmentioned part of the puzzle... I use StarLink as my ISP. They use CGNAT on their system so accessing local devices with port mapping is not really doable with out a LOT of configuration. But, they do support (more or less unofficially at this point) IPv6 addressing.
What I originally did was to past into the Safari browser the IPv6 address for my local MacBook to verify that I could in fact reach internal devices using IPv6. And happily, that appears to work just fine and I got the "It works!" page displayed.
(Yes, I know that's internal to my LAN and router, and I will test it from "outside" later to validate the StarLink routes, but at least the IPv6 addressing works)...
Then, just to verify I looked at my local file (/Library/WebServer/Documents/index.html.en) and the message in that file was not "It works!" but rather "It still works! Really no kidding...".
That made me scratch my head and wonder where the "It works!" was coming from. A different machine maybe?
I was actually trying to validate the IPv6 addressing to see if it works....which it does...but I probably received a cached version of the old original page...
As of now, I'm pretty sure the IPv6 urls are working and will load thru StarLink (which is a cool and simple way to access local resources without DDNS or Static IPs and port mapping changes)...and that I was simply dealing with initial cache issues in the browser (or Apache) associated with the first time usage of IPv6 vs IPv4. At least that's my assumption...and I'm moving on to other things...
Thx to those that replied...sometimes just talking things thru helps force a different thought process....