I have not succeded in making an ability with feather.
If I make the feather ability and use AddSpell to get it I have my amount of feather subtracted from both current encumbrance and total encumbrance.
If I make CE feather spell it works like a charm. It subtracts only from current encumbrance.
Please tell me if this a game bug.

It's a bug, all right. Aside from using a "Drain Strength 100 Points on Self" spell to fix it, I don't know what can be done CS-side to correct it.