Can anyone help me figure this out?
I thought it was totally absent, which is why I went to Tools ► Customize and looked for it, but once I found where it would normally be put, it was apparently already checked! Yet I don't see it... I have no idea of what to do.
And no, it's not in the drop-down arrow by the "U," which is only more customization of the underline.
No need to apologize; it was worth checking! But yeah, I found that odd that there was an arrow despite more space to the right until I realized that it's for very rare use cases of the underline button.