It is not neccessary to set a specific priority for a task. Only the priority order between tasks is important for scheduling.
In your case, the docs https://www.keil.com/pack/doc/CMSIS/RTOS2/html/group__CMSIS__RTOS__ThreadMgmt.html say:
"Reserved for ISR deferred thread. This highest priority might be used by the RTOS implementation but must not be used for any user thread."
So, you should not use osPriorityISR for your tasks. The prio naming may be misleading.
You can cross-check with the sources, osPriorityISR is not used in the STM32 FreeRTOS based implementation, except for some out-of-bounds error checking.
hth
KnarfB