I met twice as figure below:
when simulation finish, halt for about 3-5 minutes (while change to new QSPICE application window and do some operation), then back to this waveform viewer.
Maybe the issue of video card?
It might be the driver. If possible, it might be better to work with fewer FFT datapoints until you have a GPU or driver update. In your plot. Looks like you could get serviceable results with 1/10th the number of FFT points.