It's a (relatively) minor bug, but yes the bug will be there on release. The devs weren't allowed to say how long we can expect to wait for the first post-release patch to fix it.
It seems to be that the game recognises half siblings as in line to the throne as if they were full siblings, even if they are maternal half-siblings and of a different dynasty (ie shouldn't have any claim). This is an issue if your mother remarries and has a child and you don't have an heir of your own, as you'll likely get a game over screen if this child inherits.
It doesn't look like a minor bug to me..
Yes, I wouldn't call that a minor bug either. You could, for example, play for several generations and decide to go on a crusade because you just had a son and consider the line of succession secure enough. After all, if you die and even your son doesn't make it, you'd be succeeded by your strong genius brother, which wouldn't be too bad either.
Your wife even becomes pregnant again while you're out sieging Antioch.
However, tragedy soon strikes as your son starts pooping blood just when you get an axe to the face. You continue playing as your dysenteric son who can just watch as your mother marries the chancellor, who looks suspiciously similar to the boy your mother just gave birth to and whom the chancellor promptly, acknowledges as his own son. Due to court intrigue well above your current capacity to understand, that boy is even considered your heir.
You soon thereafter die while pooping out your bowels and get a game over.
Even if that exact scenario is impossible due to your half brother being considered a bastard who can't inherit anything no matter what, there are small possible modifications to it which can lead to a sudden, un-fun game over outside of the player's control.
And even if you disregard game over scenarios, you can no longer play the intrigue game right.
Consider, for example, playing as Leon. You just inherited Castile by murdering the former king, your brother. Now you have your eyes on the kingdom of Galicia, which is ruled by another brother of yours, with you yourself being first in line. You set up a murder plot, but while you are preparing, your murdered brother's widow gives birth to a posthumous son of that brother. Due to gavelkind succession, this nephew of yours inherits Galicia after your brother dies in a failed attempt at becoming Spider-Man. You are next in line, though, and happily start plotting again. However, unbeknownst to you, the widow of the brother you murdered first, and queen dowager of Galicia, marries a random Italian noble and gives birth to another son. Your plot fires, the toddler king of Galicia dies and that random Italian baby inherits Galicia, placing the title well outside your reach by means of succession. Sounds un-fun for an intrigue based character, right?
Luckily, such a contrived scenario would never happen.
Except that it did, yesterday, in literally the first official CK3 stream ever, and thus in 100% of all public games to date!
Edit:
Nah, I clearly remember a dev on Discord saying that they can't make any more changes for release because the game was already sent to 3rd party partners (id Steam, Xbox sub, etc).
Well, ok, I guess that settles it, then. I just hope I'm overestimating the problems caused by this bug, and for a quick patch.