Considering the fact that they’re all grouped under “Midlander” and all share the same historical origins, I would argue that Midlander Hyur are all 1 culture and it makes sense for them to share the same names. They all speak the same language, live in just a handful of cities, and make up half of the population of the whole continent.
Meanwhile the separate races of Eorzea are proudly defensive of their heritage and do not mix with each other so it makes sense that they’d all maintain their naming conventions even after Midlander Hyur (re)incursions into the region from Ilsabard in the early Sixth Era. Highlander Hyur came from Amdapor and Mhach so it still makes sense for those two populations to have different naming conventions from the Midlanders who originated from Ilsabard as well.
The reason everyone in the Far-East has the same naming conventions is because the local Roegadyn and Au Ra fell completely under the sway of the Hyur culture there.