Left Shift + CTRL + Tab not working with external keyboard

I am using an external mac keyboard with my mid-2015 MBP. When using the external keyboard, it cannot seem to execute Left Shift + CTRL + Tab. For applications where that combination should go to the previous TAB it does not work (such as a browser). If I use the right Shift instead it works fine.

The internal keyboard on the laptop works fine with Left Shift - Caps Lock - Tab.

I have been using this same setup for almost 2 years, and have never seen this issue until now. I do not recall making any configuration changes before noticing this issue. An OS X minor version update may have been related.

My computer is a Macbook Pro 15" Mid 2015, currently running OS X 10.14.5.

This is a continuation of a locked issue: https://discussions.apple.com/thread/6514636