Alibre defaults to a 0.5" fillet for English units, but you CAN change that. Or are supposed to be able to.
Generally, if you select the edges, and then edit the 0.5" to whatever you want, it works fine.
If you do the reverse, setting the fillet, and then selecting the edges, it may revert back. However, if you set the fillet and then click in the window for selecting edges, the edit should be maintained.
If that is not acting correctly, then check to be sure you are doing things right.