Decay in Preseason

So I was in Diamond when the season ended (and, still am in Diamond), and I was under the impression that there is no decaying during preseason. I got a notification saying that in 3 days I will become inactive in ranked, which will cause my LP to decay and subsequently make me drop tiers. I am wondering if this is a bug, or if I do have to play in order to stop decay, or if this notification means nothing at all! Any information would be helpful, please and thank you!