MP3 multitrack mixdown export timed selection/selected clip not working

Hello I have come across an issue in the latest version of Audition CC 10.1

When I attempt to export my multitrack mixdown to MP3 on a timed selection (After hitting IN and OUT points on the track) the file created comes out as a random length. This also happens even when I export to mp3 as a selected track. For example:

My original track is 1.06:34 (hr/min/sec). I have chopped it up in multitrack mode and created an IN and OUT timed selection ready for export this is 0.37.51 (hr/min/sec).

When I go and check my exported Mp3 file the length is 1:23.00 (hr/min/sec). There is just loads of silence on the end of the clip.

I have checked that there are no other tracks floating around at the end of my multitrack, I have tried exporting a different file type eg. WAV and it works fine- references the IN and OUT timed section. So this is only on MP3.

Tried it on a different computer (one mac, one PC)  same issue.

Can anyone help?