Why Vectored Interrupt Controller IRQ -1 ?

Most OS relies on a few hw interrupts for scheduling tasks and performing other time critical tasks.

As I said before, I believe uC/OS only uses system timer and network interrupt.

All other application interrupts are actually sw emulated by OS which calls the proper isr when the hw event occurs; so these IRQs works in polling mode, instead of using interrupts controller and the irq table. The application tasks themselves are indeed executed as part of the timer/scheduler isr

Your application can override this and request 'real' interrupts to be serviced, like you did. However this will never work if the OS IRQs (thus the application task which is supposed to be interrupted) have higher priority.