For the record, this usually isn't an issue ... unless you happen to be running Essence's Archmage skill, in which case it can be a dealbreaker. As far as I can tell, at some stage during the save/load process the game checks the player's current MP against the maximum amount their equipment and skill levels should let them have. The player's active buffs, however, are not applied during this check, which leads to ... problems. On my current character, for example, 200 of my 286 max MP are provided by a maxed stack of Archmage's True Arcane Glory proc. Every time I manually save or load the game -- or the game decides it's time for an autosave -- any mana above my 86 "base" maximum simply disappears. While I understand this is a bit of an extreme case, the autosave sucking away mana like it's the Ley Line Walker capstone is a bit off-putting. I think this is also vaguely related to why the initial version of Essence's Kung Fu skill (which had an "HP gate" for switching stances, to discourage frequent switching) would appear to spontaneously kill players "due to a buff wearing off" -- the stance wasn't applied during the analogous HP-versus-max-HP check, then when the stance was reapplied at the tail end of the save/load process, the "HP gate" ate what was left of the player's hit points.
