Our robot is less than 18" at the start of the match. When the shooter tilts up to fire, however, it breaks the 18" plane by a small amount. In my opinion this is not “expanding vertically.” It is merely tilting to shoot. I could imagine judges seeing it differently, however. Are we safe, or is any breakage of the 18" plane “expanding” regardless of the intention?
You are not safe. Robots are not permitted to expand beyond the listed to size constraints, there are no exceptions for tilting your shooter.