we are using SEPM to push client upgrades to SEP 12.1.6318.6100

but on few clients, we are getting "Trial License has expired. All content downloads will discontinue on 9/14/2015."

we never had Trial versions of SEP, issue will usually go away after reboot or update policy.

Symantec says its cosmetic, but still annoying and wanted to see if anyone else has seen this.

I've not seen it personally, but, others on this forum have brought it up. It should be fixed but has yet to be, unfortunately. I guess it is what is it for now.

You had any trial license first and then upgraded the license? 

After installing a Symantec Endpoint Protection 12.1 client, you receive a warning that the trial license has expired even though the SEPM has an active non-trial license.
https://support.symantec.com/en_US/article.TECH171252.html

no, we never had Trial versions. never seen this with prior upgrades from 12.1.2 > 12.1.3 > 12.1.4 > 12.1.5, only now with very few clients going to 12.1.6.