As seen in the linked video swapping between 2 chest pieces with reserves manually lets you keep all ammo (in this case 25 for still hunt and 9 for apex predator). However when swapping between loadouts that also both have a reserves mod equipped, the extra ammo from reserves is lost. It is not shown in the video, but you also dont lose the reserves ammo if manually swapping all of your amour pieces, subclass or other mods. This seems to only occour in loadouts.