Mortars placed inbounds do not force players to move up when sectors shift.

So in Breakthrough, you cannot place mortars inside of your HQ zone, it must be in the contested area.

However, if you have it placed, and the zones moves forward, it will not force you out of the mortar, and you will be able to remain in it, as well as being in the safety of your HQ region. With good placement, this will prevent enemies from having line of sight of you, and because the region is Out of Bounds for enemy drones, they cannot destroy it. Thankfully, if the zones move once more and you would be completely out of bounds, it will start the timer to force you to move or die.

That said, the reason I am reporting this as a bug is because it allows players to use something in a way that the game does not permit. If the mortar was supposed to be usable while in HQ region, then it would allow them to be placed there from the start. Because this circumvents that, I deduce that it is unintentional and a bug.