Reported on Something Awful.

Astral Vapors is an illness added by Tamriel Rebuilt (originally part of Oblivion) that adds Stunted Magicka as one of its effects. Curing Astal Vapors also "cures" the Stunted Magicka effect from the Atronach birthsign.

Aside from changing the effect of Astral Vapors, it should be checked if OpenMW has the same problem as the vanilla engine.