So after 6 orders total, and a lot of money spent on troubleshooting this issue (since neither SE nor grubhub seems to want to help), I figured out something.
The grubhub app takes the code, while the website doesn't. I replicated this with someone else who was having similar issues, and the ENDWALKER code was taken on the app successfully.
To help confirm this, I tried yet another order with the promo code and it said it is no longer accepted. This is something the website never did. The app receipt made no mention of the promo code, but it was itemized on the emailed receipt for $0.00.

So I spent far too much money in order to try to help as many as I can. Please spread the word to use the app, not the website.
I'm not exactly happy I had to spend so much but if I can help others not have problems, it's worth it in the end.