[TF-M] SP thread priority when servicing secure interrupts

It appears that when a SP is servicing a manifest-specified secure interrupt configured with a priority equal to or lower than the pendsv interrupt, the processor is operating at the priority of the pendsv interrupt.

While the SP is servicing the secure interrupt's signal (ie: in thread mode), If another interrupt is triggered of the same priority or lower than the pendsv, that new interrupt is not immediately vectored to. Instead, it is vectored to AFTER the secure interrupt's stub function (generated by the template) returns.

Is this true and expected behavior?

Unless BASEPRI is set to some non-zero value, I thought the processor was always running with no exception priority while in thread mode.

Alan