Since I updated to the latest version of AI (24.3) my operating system will support (Mac Pro [late 2013], Version 10.13.6), I keep running into this issue. After I create a rectangle using the rectangle tool, which I always keep at 1 px weight, sometimes the line weight randomly lowers to a percent like 0.63 px. This has happened when I duplicated the rectangle, or changed it's pixel dimensions in the Transform menu.