Associating a hot key with my ribbon tab not working

I created a Ribbon using designer. There's a tab Tab1. I added 'a' (without quotatioin) as its KeyTip. If I run add-in, and press alt, that tab is associated with hotkey Y1 and not a. How to make keytip working please ?

Before I used a as keytip, I ran Word and made sure a isn't used by other menu items. I'm using Word 2016 and there are no other add-ins except mine. Word is default US installation.

Thanks