While trying to see if I could reproduce this, I noticed that I received an email notification after moderating and then approving an old post that I was using for testing. It makes sense when posts automatically go into the moderation queue and you later approve them, but if you're testing something, for example, and moderate a post that was automatically approved to begin with and then approve it, it doesn't seem to make as much sense to receive an email notification for it.
Ultimately, this is a design issue. We assume that when content is being approved, it's for the first time. For the most part, this is a reasonable assumption and I don't think I've heard about this causing issues in production. There is potentially some tracking that we could add to detect if a post was visible at any point, but given that we haven't heard of this being problematic in production, I don't think it's currently something we'll plan to handle.