Oh! I call them spider-dice or fusion dice. And you can't collect them because they are always unlocked, just in the spider event. This is to make sure that the game still is balanced, lore-wise it might just be that this kind of die can only be acquired by a god-like being like the spider.
And as for the glitch, I have no idea, mainly because I only play with 5 die MAX. And I'd have to say that maybe it is a bug about you obtaining H a die when you weren't supposed to. How did you get the final die?