What really happened is that Hudlin started his Who is the Black Panther story and it was supposed to be out of continuity.
It was liked so much that Marvel made it the new origin for Wakanda and BP and retconed the old one away and Hudlin had to make it work and somewhat gel with past stuff.
From there, its mostly fan theory. The best one is that the Herb puts you before Bast. Bast judges your worthiness. If you aren't worthy, you die.
That kinda gels with past stuff as well. There was multiple mentions in McGregors and Priest stuff about the herb strengthening the bound between the BP and the Panther God. So you can stretch the imagination a bit and say that connection = Basts judgement.
The only time int he history of the HSH where we have seen an adverse reaction is Killmonger though. Spider-Man ate it fine. Apparanetly Zawadi did as well, althought I can't find an on panel reference to her actually taking it. T'challa didn't even know what was about to happen when Killmonger took the Herb... remember, he didn't want him dead because then he became BP by default which was an embarrassment. Hunter seemed to have hinted he knew what was about to happen though. So... was Killmonger judged by Bast and deemed unworthy? If so... why did he not die? He was just put in a deep coma. You are telling me a God couldn't kill him? Maybe the god only put Killmonger in a coma so T'challa could retain his dignity? This is pure fan theory sh*t though.
So, killmonger is the conundrum really. So... you could try and fan theory away Killmonger's getting sick which may be easier than anything else. Maybe the stuff he did to enhance his strength and speed didn't mix well with the HSH and caused the reaction? Maybe Hunter poisoned it and it caused that reaction? Killmonger is the one that stated the line of Bashenga is the only one that could handle it... maybe he was just freaking wrong lol? How would he know but no one in the royal family or close advisers knew?
I've been reading waaaaaaaaaaaaaaaay too much on this lately lol.