Does this still happen if you add "STMMaskableGraphic" to the gameObject? If it's already there, does it still happen if you remove it?
Is this happening with The Universal or Ultra shader? Is there a difference between the two?
Also, please try toggling "ZWrite" off if you're using the Ultra shader!
So is this not happening with, for example, a default Image component in place of your map hexes? Are you doing anything with the zBuffer or rendering order with your map hexes...? Or are they *only* composed of masks and polygon collider 2Ds...? If it's just happening with that object, it's got to be some conflicting interaction between them...