Ah .... so its not supported in a HA pair. Hot dang, I did not know that.
But even just searching this forum for that "topic" reveals that that is the issue *cry*.
Ow well ... .. do we REALLY need HA anyway ... ... . ...
Thanks for pointing me in the correct direction.
PS: I was looking at the configuring IPv6 on MX documentation page, and it was not mentioned there, BUT .. it IS however mentioned on the "IPv6 Support on MX Security & SD-WAN Platforms - Core Fundamentals" page.
Maybe I have gone blind during my vacation, who knew that could happen 🙂