I'm looking for a term (or terms) to describe a person who leaves a group (or organisation), and then later re-joins.
Something like a "re-joiner" perhaps?
For example, a "leaver" is someone who leaves or has left, and an "onboarder" or "inductee" may describe a new-starter. My language is British English, but it doesn't really matter.
Example sentence as per the comments, "Mr ABC is a _____; he left in February but returned back in May". Or, the actual usage in my own field:
Person A: "Is he a new starter or a leaver?"
Person B: "No, he's a _____; he left, then came back two months later"