this works for me… are you sure your 2’ edge is exactly 2’ and that your fillet radius is set to exactly 1’? when i try that, it works.
that said, i’d personally just draw the 180º arc and trim the rectangle with it… using Fillet will leave you with two quarter circles instead of one 1/2 circle.
I usually use this, Arc Tangent Tangent Radius, with just 2 curves and yes Rhino does the radius! I use it a lot when drawing out my guides for 3D. Once you have your arc between 2 curves hit enter. I have an Alias for this I use it so much.