Let's say i equip a runeblade that comes with the obliterate spell. I am able to use that spell whenever i have sufficient runes and don't have any spell book equipped. However when i equip any spellbook apart from the tome of eternal winter (which also has the obliterate spell), the runeblade's obliterate spell gets replaced by the first spell of that spellbook and the runeblade is no longer able to use obliterate.
I used the runeblade and the obliterate spell here as an example but this also happens with other DK spells and even other RPG series classes mods.

I am not sure whether this is a bug or intended behavior, could you please clarify?