Surely naming conventions in fantasy worlds can be whatever they want.
Well, of course they can, but the writers of the setting acknowledged that they had just pulled together a mediaeval fantasy soup and used that as their basis. It's why the dozen or so great houses that have existed for 1500 years with the same dynastic names intact are referred to modern US fashion - Darien Avan, Prince Avan or even Prince Darien Avan (shudder), rather than Darien, Prince of Avanil.
There
is one line in their setting book which says that noble titles shouldn't be used simply as an extra name, but the writers largely ignore it, which
is still irritating, but they did admit to simply not doing the research and later books were much better. It also features the same centuries-old families without a single ruler sporting either a byname or a regnal numeral, because of course in 1500 years not one family has ever repeated any name for their generations of offspring.
As Plotinus says though, most of that can be excused (because it's a poorly-researched fantasy setting), but the
really infuriating thing about the whole affair is that the setting attracts both history buffs and those who are (to put it politely) very sure of themselves, so when the two collide, you get highly-opinionated people with a slight knowledge of whatever they're warbling on about anjd then
still making the same stupid mistakes (Prince Darien Avan, Archduke Aeric Boeruine etc.) whilst they're busy ranting about how feudalism did this or
droit de seigneur did that or how mediaeval monarchs were sovereign autocrats with no checks on their power etc. Raaarrrgghhh!!