Hey guys, 1AM Spara here with a mental breakdown.
So, with Engage's OST, there are multiple versions of the main map themes, each with a bit more complexity than the previous.
However, one thing I did not realize is that the game actually shifts music tracks midmaps sometimes - when you break down the door in Ch10 the music "upgrades" and adds the choir and some bass simultaneously to the soundtrack while it remains the same piece. I did not notice it the first time, but holy shit this is cool. It's kinda subtle but really cool in practice once you notice it. FE has done midmap soundtrack changes in the past, but nothing like this, and rarely are they permanent like this one!