In the quickstart link: https://www.backtrader.com/docu/quickstart/quickstart.html#the-broker-says-show-me-the-money
TestStrategy.notify_order is missing the following at the end:
# Write down: no pending order self.order = None
Without it the buy order stays open for the entire session.
There has been an update to that part and probably that got missing. Thanks for spotting it.