I think it's not that you're getting worse, it's just that certain things are progressing and certain things aren't. You've definitely learned a lot about color technique: the shading on 2019 is astounding. ^^ But your anatomy hasn't actually changed that much since 2012. If you want a more dramatic improvement next time around, I'd recommend working on that.
Also, I think 2016 might be throwing you off; it looks a lot more "uncertain" than both 2012 and 2019. And that's okay, everyone has experiments and rough patches. Just don't let them fool you into thinking you're regressing when you really aren't. ;]