I'm a long time AE user. I've never seen this issue.
When I start a composition to fully render, it's fine until I jump to another application and let it work in the background.
As soon as I click off of AE, I get the "job finished chime" and it shows a completed render up to the point that it was at at the time I jumped off.
On a side issue, but might be related, AE "auto hides" when I click off to another application, rather than stay on screen.
I'm working on an iMac with Yosemite 10.10.1 OS.
AE CS6 and later versions can all run on Yosemite. Each has its own problems with the OS. If you haven't updated AE, you need to do so to get the fix for Yosemite.