Seriously - am I the only one who put 'too dear'? - only to be told that was the wrong word? It's what we say, which is also why I put 'beside the water' rather than 'by the water' for the last one - also wrong according to the pedants who should check out what people actually say rather than demoralise folk with loss of 'Health' if on the App.
It's not pedantry. Duo's courses teach standard American English. While every effort is made to include dialectal variants, it's impossible for the native speakers who create the courses to be familiar with every English dialect, no matter how common terms may be to us.
Even after taking dialect into account, when dealing with thousands of sentences with dozens of possible translations apiece it's inevitable that some things are going to slip through. That's why we have a report function.
I sympathise (and I agree that your answer should be okay), as I've been the victim of this myself, but the best thing you can do is use the 'my answer ahould be accepted' function within the sentence itself. That way, it goes straight to the moderators.
There is also the fact that Pearson's collaboration with Duo is causing some interference with the non-Pearson courses. It could be that.
Out of curiosity, what was the exact German sentence where you couldn't use 'by'?
I think the sentence was 'wir essen bei dem Wasser' and I used 'beside' rather than 'by' as I was going for what I would say, rather than a literal translation of the words. In the discussion about that, someone had asked what 'by the water' even meant, and was told 'by, as in beside'! That is why I made my comment on this page - usually I just read what others have said and leave it at that, as other people have already said what I would say. This time however there was no feedback on the page, hence my indignation!