it is not a consistent drop from 11 and higher, just apart of the loot pool. 

I keep getting the message that I got immortal soul essence in my loot, but they aren't turning up. That's not the same as appearing in the loot table and not getting them. Today is is the 2nd time, and no, of course I don't have a screenshot, because I'm not in the business of screenshotting my every move in a game that I play. I'm not asking for the stuff I lost to be replaced. just fix the bug please

This is the first I've heard about this, so it's something I would want to see documented, preferably via recording.