I try to find the problem.
It seems to be because fxsound thinks it’s a 5.1 channel device, but doesn’t output all of the 5.1 channel sound to my sound card (Scarlett 2i4). Because my soundcard is 2-channel, it loses the sound information from the back channels.
I tried adjusting the channel of the FXsound Audio Enhancer device in windows sound from 5.1 to stereo, but it automatically changes back to 5.1!