I have a skeleton spawner that generates skeleton mobs. I kill all but one and put a nametag on them and leave, going at least 128 blocks or more away. Sometimes they stay, sometimes they don't. I logged off for about 12-16 hours and when I came back, the named skeleton was gone again. This is on 1.12.2 Survival MP. Server is set to Hard mode and is not switched between modes.
Mike Yust
I have a skeleton spawner that generates skeleton mobs. I kill all but one and put a nametag on them and leave, going at least 128 blocks or more away. Sometimes they stay, sometimes they don't. I logged off for about 12-16 hours and when I came back, the named skeleton was gone again. This is on 1.12.2 Survival MP. Server is set to Hard mode and is not switched between modes.