Orbi NAT Hairpinning/Loopback Not Working

I can verify that NAT loopback works on the (older) RBR50 Orbi.  Some possibilities come to mind:

• What port is being forwarded? I have a vague memory of certain ports being "special" to the Orbi router. (my memory is not that great.  On my RBR50 the 'drop down' menu for port forwarding includes http (80) does not include https(443).  Are you trying to forward 443?
  i.e. does the specific port number make a difference?
• What DNS service is being used to resolve DNS names?  I just now attempted to reproduce this error on my RBR50, by forwarding port 80 to one of my Pi-hole servers.  My PC is not using Orbi to resolve DNS. (using CloudFlare and Google instead). Got some goofy error message that access to http://<my-DDNS-name> was denied.  What this appears to be was that somehow the DDNS request got 'NAT-looped' to my Pi-hole, which threw a fit. I swear that I have done this exact experiment before to demonstrate that NAT-loopback 'works'.  Now, I am confused (and unhappy).
• Can the problem be duplicated on another Orbi 750?  Alas, only some nice volunteer who watches the forum and has a 750 could do this.
• The 750 firmware may be broken.  There have been plenty of issues raised about the 750 firmware.  This might be yet another problem.  (Do I remember correctly that there is 'beta' firmware going around for the 750?)