Hey @TehWan,
Thanks for posting such descriptive issue.
To check the problem out I first did some basic tests using gr-limesdr and NCO. My basic test flowgraph was Noise source->LPF(500kHz)->LimeSDR(TX) on transmit side and LimeSDR(RX)(different board)->QT GUI sink. As expected, the whole signal bandwidth is 1MHz and when I set TX NCO to 1MHz signal moves to the desired location without change of bandwidth.
After downloading and trying it with your provided signal I noticed the problem you were describing, meaning that the bandwidth definitely increases when using NCO. Looking at time graph of your signal though, it sometimes gets above [-1;1] range and multiplying signal by 0.8 fixes the bandwidth problem(see screenshots below).
With full range of your signal, NCO on RX side and Xlating on TX side:
When your signal is multiplied by 0.8, NCO on RX side and Xlating on TX side:
Regards,
Richard