Dragonfire hits for 10 instead of a max of 5 damage

What I did: fought black and blue dragons, got hit for 10 with anti-dragon shield on

What I expected to happen: to be hit for maximum 5

What actually happened: got hit for 10 with anti-dragon shield on

IDs of related NPCs/items: anti-dragon shield: 1540

2009-era source (if relevant): https://runescape.wiki/w/Black_dragon?oldid=1490796

"Even with an Anti-dragon shield, dragonfire will still deal up to 5 damage, unless an anti-fire potion is consumed."

Screenshots or video:

If the bug is exploitable make sure you tick the confidential checkbox below