The unfortunate thing is it's only the default Viera hairstyles that don't include the hat hair models. The 20+ hairstyles they've added to the race since Endwalker's launch all have the existing necessary assets to enable functional hats. Of course, this is the type of thing I'm sure they would rather do all at once rather than implementing Viera headgear in some bizarre state of half completion where 2/3 of hairstyles function with headgear and the remaining 1/3 don't. That would be even weirder than what we already have where there's at least some consistency with what shows up and what doesn't. When it's fixed it'll probably be fixed all at the same time.
Also echoing the thought that I wish they would more selective when it comes to fitting circlets and bows. They don't test them on enough hairstyles or races and we end up in a situation where even out of the maybe 15% of headgear in the game that works on Viera most of it clips, floats or clips and floats.