Okay, my conspiracy theory:
Perkins is the AP guy, right? That's his job.
Except he wasn't involved at all with Tales from the Yawning Portal. That was Mearls and former WotC employee Kim Mohan, likely brought in as freelancer.
Making the storyline adventures takes a LOT of work. There's a reason Paizo has two teams that alternate APs. Plus, they have to start them a year in advance.
The summer/fall storyline is the big one. It involves the Neverwinter videogame and is tied to the minis. So that one's locked in well over a year in advance. They needed to start work on the concept art to send to Cryptic and WizKids in mid to early 2016. And 2016 was a busy, busy year for Perkins. He had Force Grey and AI: the Series and the three convention games (one more than 2015). Then the winter AI special. And he was doing Dice, Camera, Action all year. And he was involved in Volo's Guide to Monsters, helping with the lore and flavour there.
I suspect Perkins was going to blow his deadline for an adventure.
There was too much and Perkins couldn't handle the winter/spring storyline and the summer/fall one. So Mearls and Mohan stepped in and did Tales from the Yawning Portal at the last minute. Because changing that storyline didn't involve tweaking minis and video game plans. Which is why they went with reprints that could be quickly converted (or not at all for Dead in Thay). And why a third of the maps are reprinted (all the Against the Giants ones). And why they put that adventure out when it overlaps so very, very much with the adventure immediately before and after.
They rushed TftYP out the door while letting Perkins double down on his work for Tomb of Annihilation.
So Artus and the ring of winter storyline got bumped. Back burnered to winter/spring 2018. Or later.
Which sucks… but is understandable. Because Perkins is human and I don't want him burning out. And it means Tomb of Annihilation is a better product. And Tales from the Yawning Portal made a heck of a lot of people happy, so win-win.