3-zone HVAC (HZ432) one zone's a/c stops working when hot in afternoon
I can imagine a case where the damper motor stops working in the heat of the day, or the heat somehow causes the damper to bind.  Those zone controller systems have a diagnostics mode that allows you to test many of the components from the control panel.  When it's hot and zone 2 isn't working, you can test the damper with the control panel and perhaps measure the voltage going to the damper motor when it's being activated.  I think there is even an error light if certain issues are detected with the damper motor.

You can probably hear the motor working (or not working) when running the tests and that alone can point you in the right direction.  Checking the voltage can help rule out a bad transformer or even a fault with the zone controller.

It's a hot job, so download the instruction manual for the zone controller and plan out which tests you need to do before getting up in the attic (assuming it's all in the attic which could be completely wrong).