For some reason thats how target lock works. If you have target lock and die, it seems to almost disregard all damage you have done previously and you lose the lock.
Yes this is correct if the group wipes all damage dealt by the group resets to zero even if the boss doesnt reset this is because they now have to put in another 15% of effort to earn the lock again. This was also made this way to prevent a group from wiping respawning and instantly locking the boss over another group. Imo its fair this way.
I disagree that it is fair. Lets say you are killing aggy and you have him down to 10% health. You have two groups, tank and healers group, and dps group. Also, a rival clan came past 70% and is dpsing in hopes you will slip up. Maybe aggy keeps dodging the taunts/warcry or the other clan is calming your tank (and sadly it happens) and the tank loses aggro for a short period, but aggy kills your entire dps group. This would give the other clan the lock even though you have not only done the most damage but also tanked the entire boss (and are still tanking it).
It is also unfair to people killing stuff solo. You can have something 90% dead, have someone else come and attack. If you die, they will then get drop. Even though you most likely wouldnt have killed it if they didnt come along that does not, in any way, mean that they should get the entire reward for killing whatever it is.
This feature only prevents from one scenario which wasnt much of a problem to begin with (at least not on the servers i play) and hurts in pretty much all other ways.
Enjoy the little things in life...
One day you'll look back and realize they were the big things.