From the UESP Wiki:
"if you are under the effect of a Fortify Attribute, and are then hit with a Drain Attribute effect for the same Attribute, then when the Drain spell expires, the Attribute will NOT return to its former value, making the Drain spell behave more like a Damage Attribute. The only way to solve this is to remove whatever was causing the Fortify Attribute effect, and then cast a Restore Attribute effect. You may then put the Fortify Attribute effect back on to be returned to your former values."
I don't know if this was a bug with the game data or the engine (in which case it would concern OpenMW), and I could not find it in the bugtracker.
This seems like it's really a bug and not a design flaw and should'n't thus be replicated in OpenMW.
Is the Fortify Attr + Drain Attr bug present in OpenMW?
Re: Is the Fortify Attr + Drain Attr bug present in OpenMW?
Oh, I remember that nasty bug.
Re: Is the Fortify Attr + Drain Attr bug present in OpenMW?
I tested and no, that bug isn't in OpenMW.