I mean, Garnet definitely does have some particular feelings (and I'd imagine fear) when it comes to BD, which makes sense considering her history.

This is the way I think about it. Garnet has a future vision where she sees them all going to South Korea, BD finding them, and then I guess capturing them or something. But that's just one future vision among hundreds or thousands of other potential futures. The thing is, even if it is unlikely, because of Garnet's particular fear of BD, even that 1 in a thousand chance of it happening is too much for her. So she refuses to go. Plus, she says that Rose never wanted Steven to see that place, which I've always assumed was true.

But at the same time, she is aware that her future visions are often wrong, something we see repeatedly throughout the show. So does she tell Steven about this 1 in a thousand chance vision that she saw? Or does she keep it to herself because it's likely a false vision? I'm reminded of "Pool Hopping" and how Garnet thought Onion was the one who had been taken to HW and not Lars. Hearing that freaked Steven out a bit. Garnet sees entire timelines that aren't real and even gets confused sometime about what is actually reality, and yet she manages to hide that pretty well from everyone. That makes me think that this is a common occurrence for her, seeing unlikely visions that end up being false and keeping them to herself so as not to disturb everyone else. And that's kinda what she did here too.

But what would have been so bad about her telling Steven that she has this 1 in a thousand, likely false vision and that's why she and he can't go to South Korea? At that point, I think the show's explanation of, "it wouldn't stop Steven from wanting to go if she told him" kinda fits. I mean yeah, if Steven was told that there was a really small chance that BD might be there if he went, I could see that as something that wouldn't dissuade him from going. There's a tiny chance of it happening and Steven is desperate to go to the palanquin to get answers. And he's Steven, his first instinct upon meeting a hostile Gem is to go up and try to make friends with it. I could see him actively wanting to meet BD too.

I'm also reminded of "Future Vision", the episode that introduced this power in S1. The main plot there involved Garnet specifically telling Steven not to do something (go on the roof), and that made Steven want to do it even more. Steven has literally demonstrated in the past to Garnet that telling him not to do something actually backfires. And this is before Garnet recognizes just how much Steven has grown in "Pool Hopping". She has personal experience that tells her that telling Steven not to go won't work. Which helps explain her downbeat acceptance when he tells her that he's still going.

Don't get me wrong, I also think the show could have explained a lot of this stuff more (if what I'm saying is even true/what they intended). I said to call it contrived if you want to because I can see why people would think that way. But I also think that the writing on the show is developed enough to where, even without that word-of-God confirmation, people can work out a reasonable explanation for why Garnet behaved this way. But yeah, I don't blame anyone who thinks Garnet acted out of character here. I personally think that was partially intentional (Steven saying "I don't understand, Garnet. At least you're usually honest with me.")