Ok, i just figured out it has nothing to do with mask specificly. The reason for me to notice that is probably because Pota/Mask combo is often played 🙂 The reason for the delay of the backline seems to be the amount of units on your board.
With 16 units the melee backline starts running later. Removing any unit (in my previous tests it was the mask, such a coincidence which was misleading
) makes the melee backline start running sooner.
Try this template:
{"Towers":[{"T":"looter_unit_id","X":4.0,"Z":-3.5,"S":0.0},{"T":"looter_unit_id","X":-1.0,"Z":-5.0,"S":0.0},{"T":"looter_unit_id","X":-2.0,"Z":-4.5,"S":0.0},{"T":"looter_unit_id","X":-3.0,"Z":-4.0,"S":0.0},{"T":"looter_unit_id","X":-4.0,"Z":-3.5,"S":0.0},{"T":"looter_unit_id","X":-1.0,"Z":-4.0,"S":0.0},{"T":"looter_unit_id","X":-2.0,"Z":-3.5,"S":0.0},{"T":"looter_unit_id","X":-3.0,"Z":-3.0,"S":0.0},{"T":"looter_unit_id","X":-4.0,"Z":-2.5,"S":0.0},{"T":"looter_unit_id","X":-1.0,"Z":-3.0,"S":0.0},{"T":"looter_unit_id","X":-2.0,"Z":-2.5,"S":0.0},{"T":"looter_unit_id","X":-3.0,"Z":-2.0,"S":0.0},{"T":"priestess_of_the_abyss_unit_id","X":-4.0,"Z":4.5,"S":0.0},{"T":"masked_spirit_unit_id","X":-4.0,"Z":3.5,"S":0.0},{"T":"golden_buckler_unit_id","X":4.0,"Z":4.5,"S":0.0},{"T":"looter_unit_id","X":-4.0,"Z":-1.5,"S":0.0}],"Rolls":["looter_unit_id","nekomata_unit_id","sea_serpent_unit_id","cursed_casket_unit_id","yozora_unit_id","eggsack_unit_id"],"LegionSpell":"","Gold":9737,"Mythium":10066,"Wave":6}
This template has 16 units on board. You will see the damage on the Pota taken is way higher than when you remove any unit, making it 15 (yes you can also remove a random looter).
A short preview of what you will (hopefully lol) see when reproducing: