Do-it-yourself Mysterium Fidei?

After I had been at my last parish for some time, a choir member asked why we never sang the "Mystery of Faith" that he liked.

I tried to use all of the (then four) "official" ones in English, (Latin was a no-go,) and used the settings from the (mostly horrible,) Ordinairies in the parish rep when I had to, and those from Mass settings I was teaching them when I could.

So I asked, which one is that?

He hummed a tune, with some garbled words and in the combination I eventually recognized the Lucien Deiss "Keep In Mind."

I just said we could certainly use the piece sometime as the Fearsome 4th (option,) but not in  place of one of the prescribed texts there, and thought no more of it.
Can't recall if I ever programmed it in some other capacity.
This parish had a lot of silly habits, wasn't sure where they came from, and no one could ever tell me where they were authorized. (There's a reason "parochial" means what it does....)

Listening to a Mass on EWTN from the Archdiocese of Miami today, and whaddya know?
They sang the Deiss K in M instead of any of the, (now different,) "official"versions of mysterium fidei.

Was this a common liturgical.... prank? (It doesn't rise to the level of an abuse, I suppose.)

Or was there some kind of allowance made to sing other songs, or insert other acclamations at that point either by our national conference, or universally?