使用gunicorn時可以顯示ptint的文字 - jenhaoyang/backend_blog GitHub Wiki
It turns out the print statements were actually getting through, but with delay.
The gunicorn docs for --enable-stdio-inheritance note to set the PYTHONUNBUFFERED, which I thought I had, but it seems with wrong syntax.
I solved it using a .env file with my foreman setup to set the variable like this:
PYTHONUNBUFFERED=TRUE