Might the cause potentially be Samus's increased fall speed making them fall too far to continue the combo?
The increased air speed should compensate for that particular change. I mean, I've still manged to chain fairs myself, though I've not spent a bunch of time researching how things may be different. There's that chance that he meant it doesn't chain at certain percents anymore? It's definitely a possibility. Even if the changes to air speed and gravity/fall speed were made to be relatively balanced, and they seem to have been, at a certain point gravity would affect Samus' vertical reach... if I'm remembering how gravity works correctly. Thing is, when chaining fair you're mostly chasing horizontally and a balanced change should still allow the chain at many of the same percents. But seeing as fair's knockback doesn't exactly follow the line of your pursuit, then I can see it eventually becoming unchainable sooner I guess, but only a little and due to lack of vertical reach if anything.
I'm just hesitant to believe the increased fall speed/gravity is entirely to blame necessarily for this fair difficulty, especially with her increased air speed. A balanced change of airspeed and fall speed/gravity should project Samus on the same angles as before when jumping forward and fairing. Even if they weren't changed in a perfectly balanced manner, the size of fair's hitboxes should compensate to allow them to still chain at a good chunk of percents like before. Now, if her fall speed/gravity was increased much more than her air speed, then you could expect her to jump at a more vertical angle and perhaps not reach her opponent due to lack of horizontal and vertical reach. But the changes to Samus here seem to desire a balance with the old numbers. Basically make Samus move on all the same angles as before but faster all around.
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There is another possibility though...
Looking at her change log, I think the fourth hit of fair is her last, but I'm not certain of that. Assuming it is though, then there appears to be an angle change and a modified windbox knockback value on the first hitbox of her 2-4 hits. Not sure where the first hitbox is on her move, but I guess if your hit with that first hitbox on the fourth hit of fair... then you can expect things to look a little different.
My guess is that the first hitbox of all the hits of fair is the one closest to her body and that the changes to it are meant to make the move hit into itself better maybe? The windbox value makes sense in that regard at least. Trouble is, people mostly hit with the hitbox further away from Samus with the final hit of fair I think, so if that hitbox hasn't been changed then this may just be something else or placebo. Hard to say without knowing where all these hitboxes lie on the hits of this move and what for sure people are hitting with. But I don't see any mention of changes to the other hitboxes of the later hits of fair. Only changes to the first hit's hitboxes. That new third hitbox doesn't seem to be confirmed as something that even exists in the later hits of fair either. Regardless, if you manage to hit with the first hitbox of the fourth hit last, then you can expect a different behaviour.
A look at the diff page might clear this all up, though I wouldn't know much about interpreting it. Or even if someone just knew a lot more about what hitboxes are where and when in the move to begin with and could explain it, then we could clear this up. That poster that provided the Sheik fair and Zamus nair gifs could shed some light as well.
Oh, and sorry about kinda textwalling there. I tend to get carried away as I think my way through a post, heh.
