ARM-CONTEXM3/4 Group priority and sub-priority preemption rules

When multiple interrupt sources have the same preemption priority, the sub-priority is equal or not, and if an interrupt is already in service, no other interrupt source can interrupt it; only a preemptive high-priority interrupt interrupts other preemptive-low-priority interrupts.

So

A high priority can preempt a low-priority process when the group has a different priority.

When the group has the same priority, the child priority cannot be preempted.

ARM-CONTEXM3/4 Group priority and sub-priority preemption rules