RTL-SDR source in GNU-Radio 3.7.1 weird behaivour at 900 ksps

Hi again. I noticed a strange behaivour from RTL-SDR source in GNU-Radio 3.7.1 When sample rate is set to 900ksps, actual output sample rate is only 300 ksps. You can check this easily connecting a RTL_SDR Source to a WX Gui FFT Sink and a RF signal generator. For example, on RTL-SDR Source set sample rate to 900000 and frequency to 144000000 Hz. On the WX GUI FFT Sink, set sample rate to 900000 and base frequency to 144000000 Hz. Now apply a signal on 144.1 MHz. The FFT Display will show it at 144.3 MHz. Apply a signal on 144.05 MHz. The FFT Display will show it at 144.15 MHz. Now on the WX GUI FFT Sink, set sample rate to 300000 and repeat the above steps. The signal will be displayed at its right places. Interestingly, setting sample rate to 1Msps works fine. Tested with both E4000 and R820T dongles. Any advice? Thank you!