THis is true across all platforms (from what I've heard). The issue is that an internal 'code' as such for some of these items is different from others. It is most annoying in things like potions, poisons, and experience scrolls. Considering that I buy all dlc (regardless of also being eso plus--I like the extras on the collectors editions), I've gotten hordes of xp scrolls that I usually end up merely destroying (at more than 1170 cp, I really don't need xp buffs, probably ever) whenever they clutter my inventory too much. My guess, from a similar game, and from looking at other errors, is that the internal 'id' of the item changes when they release things, and if that internal id number doesn't match, they don't stack. It's a shame (and has caused problems with an addon recognizing my regular soul gems, simply because the ones I have on some characters actually came from a time prior to 1T, and so were 'altered' and have a different code than what you usually have for soul gems)...in my case, the soul gems stack, but the internal id isn't recognized by the addon that counts my soul gems, sometimes leaving me wonder if I somehow got rid of the ones I had...one of these days I'll have to fix that, by changing them out for ones that do register.

@Alassirana