YOu have not specified if the micro goes into the interrupt.
What I read from the Interrupt chapter is that – Level 0 can not be written (I1_x=1, I0_x=0). In this case, the previously stored value is kept. (ex-ample: previous=CFh, write=64h, result=44h) Can you pls comment writing into the ISPRx registers and let us know the behaviour ?