This is a bug with targets present since some time. I think it was something related with my branch but now I see that it isn't.
The bug happens sometimes to only some players. You select target player on the group menu, but you summon another player. Because you are really selecting that player. How to test this? With other player with "Target of target" option enabled, select the summoner. You can see that he isn't selecting the player he spects. And, if he move the mouse over the other players, they are selected automatically. So, you don't need to click on a player, you pass the mouse over him and the target changes. The summoner doesn't see this on his target frame, but it's happening. Really strange.
May be I haven't explain well the bug, english isn't my native language, I hope that you can understand it.
Greetings.