I posted this problem on the
beta forums almost 3 weeks ago and confirmed that it was caused by each one of the following 3 addons that I use (out of 158 addons tested). Namely CTRaidAssist, LaVendettaBossMods, Titan BG.
ie: disabling ALL addons and turning on any one of these displays the issue.
A 100% repeatable display of the issue can be seen as follows :
DO NOT TARGET A MOB, walk into its aggro radius and aggro it WITHOUT EVER TARGETTING IT. You will enter combat but be unable to see its target frame. If you target it first the frame of course appears as normal however you cannot change to a second mob once in combat, as has been reported by hunters with their pets etc.
This issue occurs when soloing or in party or in raid, ie: its not a raid specific thing as the affected mods would suggest.
A very brief test at lunchtime today suggested that patch Update 11 of CTRA from
www.ctmod.net may have fixed this problem using CTRA. I have alerted the author of Titan BG to the issue and he has acknowledged he can repeat the problem and so is diagnosing a fix. I am not sure if LaVendettaBossMods was loaded when I tested the issue at lunchtime.