The reason you can respec at anytime is because you are a doppelganger, who is tricked into thinking you are who you are- I wrote a theory about this a few months earlier, can find and read it under my profile. Dopplers abilities always are what they mimick, a doppler that is connected to a mindflayer hivemind has the ability to pull from any source of hero abilities and specs that they have ever enslaved. The tadpole is that direct link. AKA when you respec it will likely be because the tadpole overwrites your conscious mind.
I mean it's not entirely out of the question but I think it's extremely unlikely and you are probably looking too far into it. The big problem with D&D in videogame form is that unlike a regular D&D game there are no other humans at the table to tell the player if he is making a catastrophically inept character and a lot of people who will play BG3 have never in their life played a D&D game. So it's important to be able to learn by trial and error without potentially having to throw the save file away and start over from the beginning every time. D&D is an incredibly complex game. So I don't think there will be any form of forced explanation there like a doppelganger plot or anything of the sort.
