Spoilers and such.
We learn in Bastion that souls don't just go to the Shadowlands, but are retrieved and brought before the Arbiter before being judged and even evil people aren't immediately sent to the Maw.
With the new short, we even see that Arthas was dumped into the Maw without being judged, implying he might not have been destined for the Maw but possibly redemption.
Now remembering that Sylvanas died and immediately went to the Maw, we now know why Arthas was there but not only that, Sylvanas didn't get judged and sent there, she was brought there by the Valkyr and brought back from there by them, Valkyr created by the Scourge which uses the power of the Maw.
Sylvanas fearing this dark fate for her and her people(mostly her) led to her allying with the Jailer and doing some evil stuff. This means that she could have been redeemed and may even still be redeemable now and may lead to her betraying Jailer if she ever realize it, or go the route of Deathwing and realizing you've been tricked but since you've gone this far, there's no going back(feelsbadfordeathwing)
There is also the possibility that this is never addressed because blizzard might not have thought of it and I'm just using logical thinking here(indoctrination theory)