Player constantly leaving & rejoining

So I just finished a competitive match with a team of 5 people, there was 6 in our team however, the 6th person didn't select a player for the whole game, they were just idle.

This went on for over 5 minutes and they still weren't kicked for inactivity. Then once we finally got the first point they get leaving and rejoining to reset the timer

What's up with this?