In my code which is written from scratch and doesn't use any libraries as such I read the RSSI immediately I get a PayloadReady indication as the very first thing it does, i.e. before I read the data out of the FIFO, because as soon as the data is read out the RX can restart and the RSSI can get overridden. I don't have any trouble with that at all. Maybe worth moving the RSSI read to much earlier on in the ISR, it appears to be read after the FIFO is emptied and the device put back into RX mode in the library. It maybe a timing issue with your code compared to Felix's.
Mark.