There is something wrong with the half point line thickness icon. Go to tools/customize/commands and drag-drop the half point outline thickness icon to your workspace. now draw a line and change it's thickness using that icon. draw another line or copy that one and give it a hairline thickness. FOLLOW THESE STEPS EXACTLY. they will be the same thickness.
THIS IS A BUG. please try this and report back.
Yep, it changes the line thickness to a hairline.
Not that I would use that button in the first place, but it is wrong.
Mike