Hi! It sounds like you changed the attachment bone for the weapon or something similar, because in a fresh project you can equip the gun while moving or falling and the ADS is always aligned.
When the gun is attached to the bone, it stays fixed in relation to that bone even though the bone might be moving, and the aiming calculations are based on this relative position which don’t change unless you add code to change it.
Check your attachment code and compare it with the original code from a fresh project and try to see what’s different… Also check if you don’t have code that moves the gun in relation to the bone or socket after it’s attached…