The following is from the PIC32 FRM -
The user-selectable priority levels range from 1 (the lowest priority) to 7 (the
highest).
So, I think 1 is not the highest priority.
Can you check that the port uses configMAX_SYSCALL_INTERRUPT_PRIORITY
to only disable interrupts upto this priority like this - https://github.com/FreeRTOS/FreeRTOS-Kernel/blob/main/portable/MPLAB/PIC32MZ/portmacro.h#L101?