Jack wasn't present at the meeting yesterday because he had gone overseas. Since then, he hasn't returned.
I too have question with the use of “since” in this context for some reason.
I am trying to see the context better by changing it slightly for comparison.
I would say, the following sentence is a mirror in structure to the posted question, except for “since”. I’D d think it’s fine to omit it.
"Jack was not able to make it to your wedding last week because he had done overseas on a urgent call from his boss. He still hasn’t returned yet."
I think the problem is on "yesterday" which makes the usage of "since" rahter awkward. That's just my opinion.