I woould not say, that this is a bug. Magicka is increased by a constant multiplier depending on your race and birthsign. Thus, if you use the item, the magicka is increased, if you remove it, you lose it again.
Health is increased at level-advance by dividing the endurance-stat by 10 and adding this to your health (with fraction rounded mathematically).
If you use now an item with constant effect, you have to keep track, at wich level advances you used it and how many additional health points it gave you, in case, you remove it again.
IMO, because of the different calculations of magicka and health, it is a wanted effect.
Chris
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