This might be a workable change (though other things are in greater need of immediate change IMO, such as fixing rogues...), but it would have to be done very carefully. If many mobs, dangerous ones in particular, become able to temporarily reduce fortification of players, that would be a big nerf to fleshys as compared to warforged.
Wf are already mathematically and game mechanics-wise the superior race in many circumstances. A change like this done without a lot of forethought could introduce some serious game imbalances regarding the races. Now it may bring up some interesting decisions about whether to roll that new fighter as a warforged or a half orc, but any other race wouldn't even be worth considering except for role playing purposes. And that would be a mistake.