Odd that it would run out before using up it's timer, I have not had that happen on a gold seal. Don't know if there is a current bug on them. Bummer because you spent real store points on them.
ya, the clock starts to tic soon as you enter the slay area before you get to a quest on all hirelings, but never have I had them go away before the timer finished. Unless of course it is a permanent hireling from a bundled expansion purchase.
I often get myself regular hirelings for mana dv's, when I see they are already 'on the timer' I put them up at the entrance to a slay area and park it. Then when I get into quest I summon it inside. That way, even if they time out, they still are active until you totally exit out.
If you have more quests to do that are in the said slay area, such as Feywild, Cogs, Gianthold or Necropolis, when you exit the quest they come with you and are in the slayer area.
Again, I park them and then call them in, but once the timer is finished and you exit the whole area back to town, they are done.
I hope you get refunded on your points.