You're getting some bad info here. It's not "normal" for engagement to dip as your subreddit grows. What is true is that the Reddit algorithm changes regularly. Your engagement will dip if the algorithm prioritizes different content or different subreddits. For example, there was a short period where Reddit prioritized conversations... until they realized having 12,000 comments on a thread meant 11,995 of those comments weren't being read, so it doesn't move their top line.
What's also true is that your most engaged users will have an attention span that waxes and wanes. You might get a user that engages with your community constantly for weeks, only for them to wander off to different subreddits. This depends on the content; this depends on the algorithm; this depends on what the user is interested in or just going through in life at the moment. That doesn't mean they are gone, but it does mean they could circle back weeks later.
Also, where you're getting your members from is also important. If the audience growth is untargeted, it's natural for them to be less interested in the topic. A good example is that r/NBATalk should be farming members from the r/NBA audience, and not the r/NFL subreddit.
