FATE Core bug?

[EightBitz]

Is this a bug, or am I missing something?

Part 1:
I create a stunt.
I set Uniqueness to "No" so multiple instances can be added.
I allow multiple ranks.
I leave the refresh cost at a default
I add an eval script with a timing of Pre-Traits, 100:

herofield[acRefrStnt].value -= field[stNumber].value

As you can figure out, I'm sure, this means that however many ranks I add on however many instances, the cost for this stunt is always 0. And this works. When I look at the top of my screen as I add ranks and instances, the "Refresh Rate" remains constant. And when I look at the bar above the stunts list, the refresh value remains constant.

But say I flip that around.
I configure the stunt to have a cost of 0, and a bonus of 1.
Then, change the eval script to:

herofield[acRefrStnt].value += field[stNumber].value

I would expect to get the same results, but I don't. The refresh rate at the top still remains constant. But in the bar above the stunts list, the refresh value goes further and further negative as I add ranks and instances.

I know I can set a stunt to cost 0 refresh, but that's not my goal here. I'm looking to make this part of a larger eval script.

[Hollis]

The refresh bonus of a stunt is added directly to the base refresh rate, instead of offsetting the refresh rate reduction from stunts.