Steps:
1. Equip "Shisui Mengu" type headgear to female midlander
2. Equip the same headgear to female characters of other races and compare.

To help explain this issue, some terms I will use are:
Volume - Talking about hair volume, and thickness.
Bud - In these ribbon-style accessories, the main circular flower.
Ribbon - The doodads that hang from the bud.

In FFXIV, hair accessories kind of have to float away from the character's head to work. This is because, with the game's simplistic modelling, it is not checking where the hair is, and just sitting at a safe area to work for most low-volume hairstyles. The user has to accept that with high volume hairstyles— as well as hairstyles with a lot of hair texture, updos, waves— will naturally clip. This is an accepted limitation of the title.

Which brings us to the "Shisui Mengu" type headgear, which on female characters are various sets of ribbons that sit in the hair. The model for men is completely different, so we will not be talking about that here. On female character, the bud sits at a reasonable distance to avoid even a lot of high volume hairstyles. The ribbon might clip on hair that spreads outwards due to style or volume, but otherwise the bud is a reasonable distance away. In general, the game seems to err on the side of safety, and it's better to have it floating in the air with the average volume hairstyle, rather than too close.

On the female midlander, the bud is either too close to the head, they sit too low, or the item itself might need size adjustment. In general, for a lack of a better term, female midlander are wide for their height, and sometimes there seems to be issues with certain headgear, even on the exact same hairstyle on women of another race, even without horns/cat ears like roegadyn.

Some examples.



Now, you might see this, and think "Well, the hair volume is too high". The bud is too close with all but a few hairstyles, even when they pull hair away from the face. Example:



For some comparisons, this is the same hairstyle on a roegadyn. In hindsight, I wish I used the same angle and lighter hair to demonstrate, but you can clearly see the bud has much more generous distance from the head:


Miqo'te is my favourite example, because, while you can still cause clipping, they sit nice and high and a decent distance away from the hair, allowing most hairstyles to not even have the ribbon clip



The longer ribbons clip with hair that spread out a lot, which feels fair.



I haven't tried them on every single race, though so far, it seems to sit too close on au ra as well (but not as badly as hyur). On all these case, the distance from the head, the angle, or scale, should be adjusted so it works with most low-volume hairstyles.

I hope this all made sense. Thank you for reading.