I think this is because npcs that spawn for a certain goal (e.g. barrow brothers spawn due to you interacting, jad monsters that spawn at the start of a wave) they despawn if you do not attack them for long enough (5 to 10minutes).

I hope that we get a longer timer or no timer at all before they despawn ^

that's wrng I was attacking it and it just leaves

that's wrng I was attacking it and it just leaves

Well if you were already attacking it for 10 minutes long then it can despawn after all that time. Were you killing it for a long time?

no it just left same ammount of time as the others