From the patch notes:
Event Rewards: Fixed an issue causing “Distinguished Guests” and “Swarm of Suitors” to only award Legendary Cores for the highest level of completion.
The event itself spawns one of the three mirelurk groups on the island itself, which instantly spawns the queen, which instantly makes you "complete" the event having failed it. It still counts as a completed event, but you only can ever get the lowest possible reward. It's been in this state for more than a year and has been reported countless times. But yes, let's adjust the reward tiers that are impossible to achieve, that seems like a phenomenal use of effort.
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