Shift key not working with function key
(Left and right) shift key stops working with function keys recently. Shift works well with A,B,...,Z to capitalize the letters; shift works well with "Command", "Option" and "Control". However, it won't work with function keys -- Pressing "Shift + F6" is identical to pressing "F6" both effectively and visually via a key casting software. Apparently this is not a hardware problem. What could be a fix for it? Thanks in advance
MacBook Pro 15”, macOS 10.14