Someone was added to my group chat but it doesnt say who added them and everyone denies adding them.

 Someone in my group chat left a long time ago and has since stopped talking to us. Last night it randomly said "they were added to the conversation" not "this person added them to the conversation". After removing them we all denied doing so and ran tests to see if it would say who specifically added someone back, which it does. We also tested if texting the same group of people would readd you, which it does not. I'm at a complete loss, how did they get added back?