Interesting! First time I have seen this pop up. The problem came from the Data Recovery component. The default parameters (or maybe parameters that you set) were not optimal. In the BER Analyzer the results show that the MIN BER can be calculated at a threshold of about ~8.3E-5 A.U. with a decision instant of ~0.53. I simply copied these settings into the Data Recovery component and got 0 Bit Errors at the BER Test Set and complete agreement.
The BER Test Set works by comparing input and output bit sequences, so it does not really “know” what the signal quality is like. You have to make sure that you have all the detection side working exactly.