I accidentally noticed the reason why this happens, even though it makes no logical sense. When you are crafting the same type of scroll and your skill rolls up into the next 10th point, i.e. 79.9-80.0, any scrolls you made before 80 will not stack with scrolls you make after that. For sure this is why, I noticed it on my journey of gaining inscription. I do hope, also, that it can be fixed.