I just tried using the Boulder while having Deadly Attack buff and the bug didn't occur
For your case, my theory is that the Boulder somehow got a Rebirth buff. And when it came back to life, the HP was set to 9999 because max HP for items is 9999. But there's not a single item/monster/relic that can give Rebirth buff to an item. Do you remember the cards you killed/used before the bug happened?
EDIT: Now I know what caused the issue. It wasn't because of Rebirth buff. It was because the boulder killed a Saguaro, which then fully healed the Boulder. We will change it so that it can't heal items anymore.