I don't think you even need a detailed RAW explanation for why such-and-such a spell doesn't work (or why you should be able to break out of the main plot by level 3): it doesn't work because that's what the game says.
Yeah, but such things bring up how flimsy a plot element might be.
The driving force of BG3 is "Tadpole in brain. Tadpole bad." yet there are elements missing from why it can't be removed easily (It already has the "It can't be removed by normal or magical means, not without killing you" thing... It just doesn't resolve the part where killing you is an impassable hurdle... Not when Raise Dead scrolls are everywhere and Gale can access True Resurrection scrolls...)
It doesn't even need much to fix it. Just have some spiritual type NPC tell you that your soul is being altered by the magically enhanced Absolute tadpole. Voila, Reincarnate and True Resurrection are no longer viable options to bypass being infested by the tadpole.
When writing a story, it generally helps if you factor in such things into major plot elements. Even more so if there's an urge to make players find a solution to a problem - Thus encouraging people to find any loophole they can to bypass any restrictions you've imposed to make the story function.
You don't necessarily have to go into the details of WHY things don't work. Only that you make it clear that non-intended things don't work.
However, the more details you provide of WHY, the more interesting it can be for players. Especially if the revelations behind things not working help guide them towards the intended goal. (For example, in BG3, the Zaith'isk doesn't work because it's not designed to work. But it leads you to the creche and the revelations about the Astral Prism and thus directs you towards the Absolute)
