In every parish I have visited in my years as a Post-Vatican II Catholic, the choir always had the concept of the “post-Communion meditation hymn”. This was sometimes music in a different style, and usually a piece that was well-rehearsed beforehand by the choir or cantor only, and no materials given to the assembly to join in song. It is sometimes called an “Anthem” but the intent is the same. When I consulted the GIRM in this matter, imagine my surprise, or lack thereof, when I found the GIRM gives no support to this concept at all. In fact the reverse is true: the post-Communion chant is to be sung by all the assembly together with the choir, while the Communion chant preceding it may be sung without the people.
So why does everyone have this reversed? How could such a misunderstanding become entrenched in parishes if it is not part of any Church document? Is it an idea proposed by some large music publishing house? Is it merely oral tradition among choir directors? When did it start? The same time as everything else?