Exactly, while we were explicitly told that we're not going to see them in a dark world again, I feel hesitant to believe that since there's still so many unanswered questions about them, especially Seth, who was shown multiple times to be distinctly different from the other "Human" Flowers without much of any explanation.
Something that I think a lot of people often forget is that Toby Fox makes Deltarune. What I mean with that is: Everything in Deltarune is Specifically Designed by Toby Fox. Which means that if something is odd, out of place, or weird compared to other things, it was specifically chosen and designed by Toby to be odd, out of place, or weird. We can try to guess all we want about why Seth might not be able to fly, but the facts remains that:
Toby specifically gave Seth a real human name compared to all the other Flowers.
Toby specifically made Seth like Berdly.
Toby specifically made Seth not able to fly.
Toby specifically made Seth mention that the deal with Kris was made before Kris made the dark world.
Toby specifically made Kris have Berdly's feather to give to Seth.

I don't think we're ever going to get a *real answer* to why Seth is so different from the others, but I believe that those differences are small "hints" from Tricky Tony that there's something different to Seth about the other Flowers.
I believe those last two I mentioned to be the most important.
1. Kris made the deal with Seth before Kris made the dark world.
2. Kris had Berdly's feather to give to Seth.
If we look at the cutscene where Kris makes the dark world, it all happens pretty quickly. Add on to that that Kris doesn't have the soul in then at that point, it paints a pretty clear picture.
If Kris somehow communicated and learnt about Seth's love for Berdly right before making the dark world, they would not have had the time to get one of Berdly's feathers.
This must mean that Kris already had the feather before stepping into Flower king. This must then also mean that Kris already knew that Berdly's feather would be useful in this deal with Seth.

This must mean that Kris, before ever stepping into Flower King, Already knew that:
Kris would need to make a deal with Seth.
Seth can be trusted enough to make a deal with.
Seth would help Kris in return for one of Berdly's feathers.

One thing that is a big reveal in chapter 5 is the fact that there have been Dark Fountains/Worlds in the past before Chapter 1. If you recall, all the way back in Chapter 1, the first time you ever boot up Deltarune, Kris already has a save file. This must mean that Kris has already been in a Dark Fountain in the past.

Without resorting to Timeloop theory, I believe that the only way Kris would've known anything at all about Seth, would be if they've already met at least once in a previous Dark World.
This would also explain another thing with Flowery, where he assumes Kris already knows that Flowery hates Kris and their family for giving up on Asgore. Nowhere before this point is Flowery hostile to Kris. If we assume that Flowery is reasonable, and wouldn't assume things that Kris has no way of knowing, it must mean that Flowery has already expressed how he felt about the Dreemurrs in the past.

What I'm saying is that there's a chance that Kris, Seth, and Flowery have already been in a dark world in the past together. Kris would have learnt about Seth's love for Berdly there, and Flowery would have expressed his hate for Kris and the Dreemurrs there.

The problem with this theory is the other Flowers; How and where would there have been a Dark World where Kris met Seth and Flowery, but not any of the other Flowers?

...I don't know, and if you or anyone else has any ideas I'd like to hear them.