I figured out what the problem was! I had my default list nested in a folder in the Reminders app. After moving the default list out of a folder the workflow started to work properly.
When I run:
tell application "Reminders"
get name of default list
end tell
The default list comes up as "Inbox", which is right.
I'm happy to keep my default list out of folders to ensure this works properly. I will say that I'm not able to selectively input reminders to specific lists that are nested in folders right now. Not a huge deal at all! (But something I wanted to share)