Originally posted by 𒆜FabzŇ:

are you sure they died, are there remains when you come back? afaik some wolves will de-spawn, if they are not breeded ones. so always keep the breeded ones and kill the initial "parents"

Tamed wolves don't despawn, not even the parents as long as those are tamed. I believe that was a bug a long time ago but was fixed pretty much immediately.