Using mouse buttons as Shortcut keys prevents hotbar assignment

Question

When a Shortcut key is bound to a mouse button, trying to assign a build menu item to the hotbar does not work.

For context, I have an MMO mouse with an array of buttons on the side.  These buttons correspond to the 1 through 9 keys on my keyboard, except for the buttons for 1 and 6.  Because of the position of these buttons on the mouse, I have them set to Mouse Forward and Mouse Back (labeled Thumb Mouse Button and Thumb Mouse Button 2 in game).  So I have Shortcut1 bound to Thumb Mouse Button 2, and Shortcut6 bound to Thumb Mouse Button. When I have an item in those slots in my hotbar, pressing the button pulls up the builder as expected.  However, when I am in the build menu and want to reassign the item that is on the hotbar, Shortcut1 and Shortcut6 do nothing.  If I want to change those respective items in my hotbar, I have to change the keybindings to something else, do the hotbar assignment, and then change the keybindings back to the thumb mouse buttons.

Comments

Curiously, I am experiencing the opposite behavior when I re-assign to any non-numerical key. I rebound the 'Shortcut 10' hotbar button to the grave/tilde button to the left of 1. I can assign the hotbar by pressing the button normally, but to use it in world I actually have to hold shift and press the key