Audobe Audition make a bad 320kb

My line-in is at 48,000, 24bits
i recorded my vinyl all is ok if i do a flac with it but, if we do a 320kbps there is a problem
I try to do a 320kbps with my vinyl, we can see in Spek that it is very low frequence, like a 160kb, this is realy bad
so i delete the Mp3 file, and i do a flac with Adobe Audition, and I redo new 320kb with the Flac but I have to take another programe this time and it’s better than the one made with Adobe Audition, a 320kbps should be over 20khz, not at less then 16khz, look my video https://youtu.be/hYYADsVsOZY