So what happened? I think it's one of these things, or some combination of parts of all of them.
1) Management asleep at the wheel
For whatever reason this team was given way too much freedom to run wild with little to no oversight except for the occasional "yeah how's the MSQ going?" and they could reply with some basic plot beats and bullet points that, isolated out like that, all looked pretty good and got the green light from the high-ups, who were way too busy working on other things now that CBU3 is developing two other games in addition to running FFXIV. Because at a stripped down level, the story of Dawntrail isn't all that bad. It's in the details and execution of that plot where all the Bad comes in. A lot of this writing feels like the rough draft that needs some editing passes to polish it up. So many of the little things that add up into big problems like inconsistent character voice, unclear character motivation mixed with other character motivation shoved down our throats at every turn, characters describing to WoL what literally just happened on screen, etc., could be cleaned up in a few editing passes. But they never were. No one caught any of these consistent problems and flagged it, or if they did, they were ignored.
2) Bad communication and/or breakdown of communication. Someone locked these writers in separate rooms and didn't let them talk to each other or anyone else for that matter. Or they didn't want to talk to each other. Or the time they had to talk together each day was very limited. Or there was some social factor that made everyone communicating clearly with one another way more difficult than it had to be. I can keep trying to make up reasons, the point is, people aren't talking to each other and it's leading to problems, because there's no clear style guide on how to write each character and an agreement on what the themes and mood of any given scene should be, hence it shifting rapidly from scene to scene, especially in the last two zones.