Arduino: Only reads serial input when Serial Monitor is open. Fails when it's closed
Answer:

This was a big rookie mistake on my part. 

When I established the connection with Python, it rebooted the device. So it never received the data I sent. The Serial Monitor apparently opens the device so the immediately serial write from Python worked correctly; which was throwing me off the trail.

I had to connect, wait for word from my device, then start writing. Rookie, rookie, rookie.