Having the same problem here.

Was about to try and use focus, for the unexpected nuke in arena. But now i first have to get my main target fixed.

Thinking about making 4 bars, 1 for each character and keybind the barswitches to the '/follow toon2 /focus toon 2' button
So i can just use /assist toon2 /cast LB.

Still hoping it's a bug though.