Incorrect magicka and fatigue recalculation
Firstly, in vanilla Morrowind, whenever your INT changes your ratio of current and maximum magicka is supposed to remain the same, so for example if my INT->magicka multiplier is 4.5x and I have 225 / 270 magicka, if I fortify INT 25pts it becomes 319 / 383. In OpenMW it just alters your current and max magicka by the same amount (225 / 270 becomes 338 / 383) making Fortify Magicka pointless and also allowing you to refill magicka from empty by putting on a +INT item such as the Necromancer's Amulet.
Secondly you cannot endlessly cast Fortify Magicka 16pts for 2 seconds in vanilla Morrowind, but you can in OpenMW, it's probably that magicka is supposed to go negative. You can test this by making an enchanted +80 magicka for 5 seconds ring, casting spells to drain your magicka to 0, using the ring, then casting a spell and waiting for the ring to wear off. If you then use the ring again it should add less than it's supposed to.
Tested on 0.36.0 with some mods, and a fresh GOTY edition of Morrowind.
(RM-2604 from redmine: created on 2015-06-02 by Ohgod Noway, , closed on 2016-08-04 by R. D.)