Unlearnable fourth talent point

[redy5]

I have bought the game on steam winter sale and since then barely ever stopped playing it. Before achieving a win on a cornac temporal warden(normal adventure) I was playing archmage nonstop (as you can deduct, being completely unsuccessfull). There was a strange occurance I ran into twice.
So I focused on arcane in one playthrough, but I needed Flame skill lvl 4 to support my damage output to some point in the game. I didnt want to use fire at all, so I planned on unlearning it later. It was when I had already many hours in the game, so Im completely sure I havent miscalculated something. So here is what happened. I went to town, unspecced everything from fire, put all but 4 point in other skills, closed lvlup window, opened it again, specced 4 points Into flame, closed the window. Went to clear Kor'Pul, didnt spec anything, didnt have any escorts, nothing. Went back, and I only could unspec 3 points from flame. I checked if I could unspec any other class talent. Nothing. So, the tooltip says you can unlearn last 4 class talents. That was a lie at that moment, because I could only unlearn 3, not only talking about flame, I checked all my class talents. Only 3 skill points. I was very frustrated, tried to google it - nothing.
Next time the same thing happened, but between learning and unlearning I got a Celestial/Light generic category from an escort, so I thought that could have been the reason. First occurance is completely unjustified for me tho.
So, this whole story is the only thing I dont understand in the game. 1 class point may not be that much, but that is quite frustrating, especially if you are a person like me.
I would be grateful for anything resembling an explanation to this strange occurance. Thank you.