When choosing File > Export, if you check "Use Artboards" and type in an artboard number in "Range", I've noticed that if you hit the "enter" key nothing happens, and then if you click "export" it will export all artboards, ignoring the range.
If you instead follow the above steps, but instead of hitting "enter" you click "export", it'll export just your artboard(s) specified in the range.
macOS Catalina
Version 10.15.7
Adobe Illustrator 25.2.1
When choosing File > Export, if you check "Use Artboards" and type in an artboard number in "Range", I've noticed that if you hit the "enter" key nothing happens, and then if you click "export" it will export all artboards, ignoring the range.
If you instead follow the above steps, but instead of hitting "enter" you click "export", it'll export just your artboard(s) specified in the range.