If a religion is bugged, in such a way playing it becomes completely unfeaseble, then that bug should be corrected, no matter how "minor" or "unimportant" that religion is. Yes the games do indeed focus on historical accuracy, and this, regrettebly, means that Abrahamic religions get most of the focus. Though regardless, dismissing Helenicism is just as bad as dismissing any religion that would give way historicly to the Christian scourge that sweept accros Europe through the turn of the millenium. The devs have supported other pagan religions, and the new earlier srtart date means Helenicism is more historicly relevant than it has ever been, and asking to have a bug fixed, to then have a whole community come down on you to say fuck you, your religion and choices in these game are irrelevant and unimportant, is just a sign of selfishness and cruelty.
NOT A BUG. Hellenism is not a supported part of the game. Full Stop.
PD put it in just so we could have a more accurate history of the Byzantine title history. That is all. They made no promise, and have no obligation, to provide any additional support for this religion. The fact that they fixed the "Demand Religious Conversion" diplo-action for pre-reformed religions was pure good will on their part.
This does not affect only the Hellenics, but also all other unreformed pagans (So, Stormweaver and others, changing it for other pagans would automatically fix Hellenic and Generic Pagan conversion) - It's a stupid change/bug. Yes, they should be damn slow compared to reformed pagans and organized religions (Hence the ''Inferior at Conversion'' in the summary) but currently only the player can use the Diviner at all when playing pagan, which is just dumb and adds a disadvantage to the AI. (I don't think they even use the diviner for other purposes like researching cultural tech..)
Its completely intentional to reflect the fact that unreformed pagans were non-proselytizing faiths. They were syncretic by nature, so converting provinces via your Chaplain doesn't make any sense. If you want to convert conquered territory faster you need to convert or reform.
Or use a mod.