I've seen this in combo vids too. Maybe the last hitboxes are keyed to only hit airborne opponents? Or the move can only deliver x number of hits throughout its entire duration? I've seen both kinds of mechanics before in FGs.
Smash hitboxes can be flagged for air/ground, it's extremely common. But usually it's just to have an identical pair of hitboxes with different properties, most frequently angle. Mythra Nair is such a move, but this has nothing to do with when the move hits people--like most, the aerial and ground hitboxes cover the exact same space.
I talked a great deal about the "front-end" of Foresight, how its frames compare to Bat Within. Now let's talk about the reward.
Spot dodging a 30 frame move, depending on the move, the character dodging, and the how the dodge was timed, is normally going to yield between a +0 and +15 advantage. Bat Within is going to sit at the high end of this spectrum, comparable to other dodges but as a best-case scenario.
Foresight is on another level. It gives over +20 more advantage than Bat Within on all ground options, or around +35.
But this is where it gets tricky. That's an incredibly number, but +35 on Mythra is worth way less than +35 on any other character. Her "advantage -> reward" curve is exceptionally flat. At +35 frames she has a few different ways of doing ~20% to you, but not really any more. F-smash with a bit of charge takes >100% to kill
at the ledge for many opponents. For contrast, if you gave Pyra +35 advantage, she can kill almost anyone under 50%.
So, it's weird. Mythra is getting huge advantages numerically, but in cases where it's apples-to-apples she is only doing maybe 2x the average punish damage as other characters. In cases where people have lethal tilts, fast smashes, up-Bs, or throws, when range allows they might be killing just as well (or sooner) than Mythra off a successful mundane spotdodge. (Despite having way less advantage)
Most of the value in Foresight isn't specifically in the raw amount of advantage, but in the way it cuts through the spacing of ordinarily safe swipes. Then it's not apples-to-apples, and Mythra is punishing things other characters can't. This impedes people just swinging blindly at Mythra, hoping to smother her with favorable trades.
Edit:
What's the range on the slowdown? Bayo could slow a zoner at considerable distance... But even then
could bait it with air weaving.
It looks to be somewhere between 5.2 to 5.5 units, identical to Bat Within. For reference, that's an inch longer than Belmont f-smash, or half of Wolf Blaster distance.
But, sometimes I don't get the slowdown and don't know why. I though I would be able to wreck Plant by activating slowdown off of poison, but it never seemed to catch him.