This happened in Operation Crimson Hammer Tier 3 and 2 actually. I brought my SS buck to T3 and it could tank a bullet and it turned yellow. But when I played T2 OCH a lumber completely broke the shield and still dealt damage to me. I know a lumber is T2 lumber is tougher than a T2 Gun puppy, but this is a whole tier difference. I have not tested this with other shields or depths but I do believe that this may need further testing (I won't be able to test this). I don't know if this can happen with different monsters from different tiers either (Perhaps an alpha wolver in T1 and a devilite in T2).