Determine source of ⌘⌥\ being an alias of ⌘C

TL;DR, somehow ⌘⌥\ has become aliased to ⌘C on High Sierra (possibly earlier). I cannot for the life of me determine the source of this.

I first noticed this when invoking 1Password Mini began producing a system bell tone. I finally started to dig into the problem today. In the Keyboard preference pane, there are no conflicts with the ⌘⌥\ shortcut. So I disabled the shortcut in 1Password Mini. Tested, and system bell still sounds. When I attempted to re-add the shortcut, I get the following:

To be clear, this dialog displays when I carefully type ⌘⌥\ — I am not typing ⌘C.

So something has apparently aliased ⌘⌥\ as ⌘C. I have tested this further. If I highlight text, click on the Edit menu, and type ⌘⌥\, I briefly see the Copy item flash, and text is copied to the clipboard.

Any ideas? This is driving me nuts. I've quit all other applications, including menu items. I've checked the Keyboard Shortcuts preference pane for conflicts. And everything has come up short.

Update: I've installed Key Codes from the App Store, and even this application indicates I'm pressing ⌘C! However there's some slight differences that might provide useful information for debugging:

⌘⌥\:

⌘C:

Update 2:

As additional information, this happens with both an external Apple Wireless Keyboard as well as the internal keyboard on the MacBook Pro.

Update 3:

This is the full log from Key Codes, including modifier keys being pressed, up to and including pressing the \ key. It indicates immediately after the \ key is pressed (which it identifies as the c key), the c key is released and a modifier change is made as well (despite no keys actually having been released).