I believe this pertains only the PS3 and PC players who use a controller.
I submitted this as a bug report and a GM didn't really address the issue but asked me to move it here instead. So...
To be clear, when you sub-target-tab to a new target (controller), the enmity list disappears for a second, then reappears, but what appears is not the enmity for the subtarget, but the enmity for your hard-target.
In other words, there's no reason for the list to disappear, unless the game is trying to calculate enmity on a sub-target and is mistakenly calculating on the hard-target.
Quoted for Eloquence, because he described it much better than I could: