It is rumored that the third book would’ve focused a lot more on the Mueh’zala plot from the second book. However Blizzard changed their direction with Bwomsamdi due to how much the community loved him in BFA. So Mueh’zala was put into Shadowlands as the secondary antagonist of the Ardenweald plotline instead. Which Bwomsamdi was most likely going to be originally.
Also iirc, the third traveler book had a different author compared to the first two. So that could’ve also been a factor. Having a different person at the helm of a series can cause problems. Just see the Star Wars sequels for example.