Yes I've gotten the same problem, it's not only on protectors though. If you apply LB before the last tick like you said, just wait a couple second before fire blasting, should fix the problem for you.
I believe it has something to do with the initial spread of LB which will be ticking a second or 2 behind the first mobs LB, and therefore will drop off the target a bit later then the main LB. You can see in the log you're trying to spread the new LB before the old LB has fallen off the other 2 bosses. So yeh, just cast an extra spell after applying the LB.
Sorry if I was a bit confusing lol.
[19:37:22.911] Protector Kaolan's Living Bomb is refreshed by Caladriel
[19:37:22.984] Caladriel Living Bomb Protector Kaolan 19065
[19:37:23.421] Caladriel Living Bomb Protector Kaolan *157036*
[19:37:23.421] Caladriel Living Bomb Elder Regail *157036*
[19:37:23.421] Caladriel Living Bomb Elder Asani 76231
[19:37:23.892] Caladriel casts Fire Blast on Protector Kaolan
[19:37:24.092] Caladriel Fire Blast Protector Kaolan *119733*
[19:37:24.171] Caladriel Living Bomb Elder Asani *36302*
[19:37:24.171] Caladriel's Living Bomb fades from Elder Asani
[19:37:24.171] Caladriel Living Bomb Elder Regail 17622
[19:37:24.171] Caladriel's Living Bomb fades from Elder Regail