tee not working.

This works as expected:

$ nodemon src/tits.py | grep -v nodemon 2>/dev/null
1.247219128924647
1.632993161855452

But when I do this, the only effect is that the file 'prime_100' gets created, but stays at zero length:

$ nodemon src/tits.py | grep -v nodemon 2>/dev/null | tee prime_100