I have seen this issue 3 times from 3 different people, but I don't have a warrior or a feral druid to test it on. Sometimes rage will deplete quickly after combat. The other night we had 2 warriors in party, and after combat only 1 of them lost all of their rage quickly. We had a bear tank that had the same issue sometimes. I remember that he logged out and logged back in and it seemed to work normally after.
Sorry I don't have more information about this. I have a recording of it happening once, but aside from actually seeing it, there isn't much more I can show or tell you.