I'm using the latest version of Adobe Audition 2019.
When I use Low Shelf Gain in the multitrack session and play it back in the multitrack it's fine but as soon as do a mixdown and playback the exported file, the Low Shelf Gain is gone. In the multitrack it's fine, but the mixdown file has no low shelf gain at that specific moment.
What am I doing wrong ???
I've just spent a little while looking at this, and I don't think you've done anything wrong at all. I can't get any of the automation for the channel EQ to write to a mixdown at all - and I can't see any reason that it shouldn't. I will investigate this with the devs - there may be a reason for it, but if it's not available then it shouldn't appear to work fine on playback, and not in a mixdown, should it?
There is a solution though - use the rack effects, and put an instance of Parametric EQ in that (it's identical) and you should be able to control the audtomation parameters in that just fine, and it will mix down correctly. Works here, anyway.
Update: This was fixed in the Audition 13.0.1 release.