I think the observation here is that not all "engagement" is equal. I really dislike Duo Lingo's pedagogy... For some reason they are opposed to actually telling you anything - grammar rules, definitions, etc - and leave you to (hopefully!) infer them one-by-one.
Have you checked out the "tips" section for each lesson? That usually has pretty good descriptions of things like grammar rules.
Also, when you make a common mistake during a lesson, Duo Lingo will often interrupt the lesson with an impromptu tip showing you what your mistake was, why it was a mistake (what grammatical rules it broke, etc), and how to avoid that mistake in the future. Those have been pretty helpful in my (admittedly limited) experience, though I suppose it's possible the prevalence of those tips depends on the course.
> I suppose it's possible the prevalence of those tips depends on the course
Unfortunately the interstitial hints do vary by course. Spanish course has stuff like that very frequently in the early lessons (I have not done the later Spanish lessons); the German course barely has them at all anywhere, possibly not at all (if I had perfect memory it would be much easier to learn the languages).
For what it's worth, I've also used an app called Lingvist, which tends to tell you the grammar rules more directly. You might like its approach a little better. (But also, you might consider getting an old-fashioned grammar book, with tables of declensions and tenses and such, and keeping it nearby while doing Duolinguo exercises.)
