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Mysticism doesn't work correctly (WOG, I don't play HOTA, so I don't know how it works there).
There is a specialist in Intelligence, Eleazar, with Expert Intelligence and Mysticism. I've provided a screenshot for clarity (the hero stands still and simply passes the turn). The Mysticism expert restores 4 points of mana per day + 30% of the maximum mana, as described in the skill. There are 5 knowledge points + Expert Intelligence = 100 mana points + 30 points from the specialization. It should regenerate 130 * 30% = 39 + 4 = 43 mana points per day. The screenshots show that it only regenerates 30% of the 5 knowledge points + Expert Intelligence, i.e., 30 points each. Then, it reaches the cap of 100 points, and it regenerates 4 mana points per day until it hits the cap. Therefore, Mysticism at least ignores the specialization. At higher hero levels (when the bonus mana is already around 1k or more), the situation is the same.