How far are you in the main quest line? I believe at a certain point before finishing the MQ, there is a dragon that flies around and won't attack you (he has other business he's taking care of) and will eventually fly off. Have you ever hit one of these that seems to be ignoring you, and seen the enemy life bar below the compass? If so, do you remember the name of the dragon that appeared? What patch version of the game are you on?
I have finished Act I and watched the dragon resurrection.
I have spotted the 'boss' dragon again later resurrecting another dragon. When he was done he flew off and I killed the dragon he raised (it had a name but I forget what it was).
I never see the life bar unless the dragon is actually hit be either myself or some NPC. That's when it shows up as a red dot also on the compass. Most the dragons I've seen never even show up as a red dot. They are not the big boss, I know what he looks like. They just never go hostile toward me as they never seem to see me (even though I guy is jumping up and down and shouting).
I'm on STEAM so I assume I have the latest patch, since I've never done anything to stop it from patching.
Note I do get a dragon occationally, so I don't think it's a bug. I just want a way to get those few that seem to just circle some and fly off.