A damage beam with 20 blocks has 200 damage per tick, one damage instance with 147.8 and one with 52.2. If an armored block is hit, the built-in piercing effect is countered, but inconsistently.
As long as the targeted entity has armor HP both damage instances are applied to the block which was hit, resulting in 25 block damage per tick: 200 total damage times 0.25 due to 75% armor, times 0.5 due to armor HP. Armor HP receives 100 damage per tick.
After armor HP is depleted only the first damage instance is applied to the block which was hit, resulting in 36 damage per tick: 147.8 damage from first instance times 0.25 due to 75% armor.
Upon hitting an armored block the second damage instance does apply to that same block when armor HP > 0, but it doesn't when armor HP = 0. I'm not sure if this inconsistency is intended or a bug, but it's definitely confusing and makes an already complicated system even harder to understand.