All advancements, whether level, bosses and even bestiary, will be lost if a reset occurs on your server. I can tell you this from my own experience on my old server, when a server reset your char will appears with same status it have before the reset, so lets say, you enter on tibia and you kill the among of bonelords requiered for unlock the bestiary, but the server got reset you will lost all the advancement you have done.
However, if the server just turned-off for couple minuts/hours the advanced of you have done will keep on your char.