Yes, it's not just TAB. The whole target system is flawed.

For example the last boss at Brayflox, if I'm tanking it with my back against the wall, and I cancel my target, I can't target it again even by clicking on its model. I have to zoom out far enough to make targeting possible again at all.

Targeting is often obstructed by invisible or small debris. A little obtrusion can make a whole enemy untargetable unless I move my character out of the way to look at that target from another angle. Clicking on the model is also more often than not useless, while clicking on the nameplate of a moving enemy often don't register.

We've complained about this since beta and it hasn't improved at all.