Just replying to confirm that yes, when a player kills a world boss, they become ineligible for loot from the same world boss again until the weekly reset, regardless of whether they received loot for that kill.
Players can still assist in killing that world bosses again, but will not be eligible for any loot from those kills.
This rule prevents layer hopping to get a chance at loot from the same world boss multiple times each week and applies to all Era, Anniversary, and Hardcore realms.