I figured it out!
I saw another post online about issues with Mojave and suggestions to: (1) activate Mail.app and (2) add a delay. I did both and that fixed the issue such that the attachment was added before the email was sent.
I experimented to remove item #1 (activate Mail.app) and the automation still worked so I think the true fix was adding a delay to allow the attachment to be added to the email before it was sent.
Below is my final embedded script in case it helps others (the red box shows where I added a delay).
Thanks!