I am using the st25r3916b chip with phase awaken, wake-up timer range of 800 msec and delta 3.Once it wakes up, I go to read the value in the register 0x3A (Phase measurement display register) in order to check which is the difference between of the stored value and the one which awakes the chip.What I can see is that the value change is less than 3.
one scenario I could think of is that you are not fast enough in reading the Phase measurement display register. If after the I_wph you take more than ~800ms then another pulse and measurement with different result might happen. But 800ms is a long time.
If unsure then please do a logic analyzer trace (SPI+IRQ). You could also post it here for us to check.
