When opening a .ui file by double clicking it in the finder the designer will first show the "New Form" dialog as if opened without an argument. When pressing "Close" on that dialog, the form that was double-clicked is opened.

This is in contrast to opening designer from the command line with an argument given on the command line. In that case it behaves correctly. There seems to be something special macOS does when opening a file by double-clicking in the finder.