This is correct. The plunder interface is available at all times and can be refreshed to see if anyone is currently running a convoy. So technically, a plunder could happen up to 30 minutes after the end of the event, since once you accept a convoy you have 30 minutes to run it.

If plunders should not happen once the event ends, then by that same logic if you haven't reached your convoy destination by the end of the event you should lose convoy as well. Better the way it currently works.