Disappearing from chat list

I’ve noticed that sometimes a person disappears from the chat list (specifically the pinned chat list) and then reappears later. Does anyone know how they’re doing this?

I was wondering if it’s the discreet mode thing, but I thought that just hid your location from the map. Is it possible to deactivate your account then reactivate it later without losing your data? Or maybe they’re blocking and unblocking me?

Thx