Maybe Harald didn't eat it,
I don't recall They just called it Harald's DF, They said it's passed down through Generations meaning He became King & then DF was in his hands, nothing suggests He had to eat it. Actually it makes more sense that No King does consume it, otherwise it means Retirement isn't an Option, you have to Die for New King to take over,
And let's be honest, these are Vikings that Usopp wants to become like, it's obvious there are gonna be some Lies involved, and Giants already said that Harald had a Prophecy that his Son wd kill him which could be just a Lie tbh,
Also if that DF is only for Kings of Elbaph & previous King must Die for it to be Inherited, then Loki should have been allowed to become King,
We are talking about Giants who were First Introduced when Two Leaders were Fighting for 100 Years lol
That's how we were told They settle their Differences, whoever is Right will be Victorious, and Rest will Honor that,
Loki is Royalty, Defeated King & acquired that DF, He has every right to Rule, it's not his Fault Giants were led to believe He is a Villain even before He was Born! Loki was clearly set-up imo, everything indicates that Giants were fed some Lies
This Arc literally started with such Concept in that Lego World, People being Fooled by a Ruler who wants to write his own Scenario.