In a nutshell, since the Guild Bank and Warband Bank can be moved around, the tabs on the side (currently on the right) are ill placed when the respective frame is on the right of the screen or lined up with the Player Bags.

Example: if I place the Bank frame flush to the right of the screen, switching to the Warband Bank causes the tabs to be off screen.

Solution: Move the tabs to the side of the respective frame based on the Window Anchor or general window placement of the frame within the viewport.

I personally have my Bank locked flush next to my Player Bags, which the tabs are inaccessible for Warband Bank showing up beneath the Player Bags.