It's the glue that adheres the window to the frame.
It was either always like that but you never took notice or its receding naturally which makes sense considering glue is part water. Think about the silicone around a shower. Over time it naturally recedes, you get a leak and then your reapply!
Unfortunately there's nothing you can do here. Replace the whole window? Inject hot glue in and clean off residue. It's very excessive for a minor aesthetic point.
