Unfortunately there's not a ton we can do about that on our end, since the Steam backend is well and truly beyond our control. Your best bet is to check out Steam's own troubleshooting guide. You'll be looking for version 2.5.1 on the About page, or for the content warnings for Oleander to have a Chapter Two section.
And I have no idea how you'd get onto a beta branch given I don't think any of them are public right now, but if the Steam library sidebar is showing additional text after the title in square brackets like "OBSCURA [beta]" or something, then you're on a beta branch that doesn't have the update. Again, this should not be possible for you, but that would be a reason why the update isn't downloading.