I think seasoned soldier doesn't shows in "modified by" for slam, mortal strike, etc., because this passive doesn't increases their damage directly. Since they are use melee damage in their calculations (seasoned soldier increases melee damage in character stats), so all these abilities already include it.
I found my old screenshots from official PTR server: http://forum.pandawow.ru/showthread.php?t=40583
Autoattack: http://i016.radikal.ru/1401/42/b4a8cde8e8fa.jpg (without spec), http://s020.radikal.ru/i720/1401/47/60b76f856a09.jpg (arms spec).
Heroic strike: http://i023.radikal.ru/1401/f8/c1a7cd2434f8.jpg (without spec), http://s61.radikal.ru/i171/1401/ef/b3427b9f4a6b.jpg (arms spec).
http://web.archive.org/web/20140705053702/http://www.db.pandawow.me?spell=78
Heroic strike doesn't have Seasoned soldier in the "modified by" tab, but damage was still increased by it.
So, this report is false and fix should be reverted.