Fillet won't fillet at half radius

 From:  NightCabbage
4584.1 
Hi Michael

So I've found what looks to be a fillet bug? (I can reproduce it - oh and I'm using the V3 beta, but the same thing happens with V2)

What I'm doing is this:

Create a Rect, 24x24
Seperate
Select the top and left edges
Fillet, radius 12

And it will fail.

12.1 works fine
11.9 works fine

But given 12 is only half way towards any limit, I don't understand why it would fail here?