3miles, the answer lays in the Folha article ( asuming what is reported is correct of course): if my understanding of Portugese is correct , basically the flight was indeed planned at 360 but ATC lost contact with the legacy and it stays at 370.
If ATC did warn the other unit ( which had the 738 in contact) is still not known. The article mention some ATC errors ( to be taken with caution )
The article mention a loss of transponder ( which could explain the non-RAs) and some " delivery tests" done during that flight by the Legacy, " without informing ATC" but that comes from "rumours" from the military.
The direction of the FL according semi circular rule is not the key factor here ( We modify this all the time ad-hoc to ensure separation or expedite traffic ) The coordination between the 2 ATC centres is .
I would bet this is where ly one of the main cause of this collision.