Robin2:
I don't know, is the short answer.Your Tx sends a value every 2 seconds. What is the interval between the Rx printing 211 and printing 0?
Maybe have a look at this Simple nRF24L01+ Tutorial
...R
The interval is zero, because it immediately jumps again into the radio.available condition and prints the "0".
If I add a delay of for example 1ms, than the "0" disappears.