Fascinating. I have only played around with D&D Beyond...does D&D Beyond prevent one from creating a character that violates some kind of character creation rules. For example, would I be permitted to make a character that had two 9th-level spell slots if I wanted to make such a character?
You would have to override the default.
If DnD Beyond allows the epic boon, it doesn't mean it's correct, but it does mean the programmers and whoever is directing them think multicasting still grants the boon.