It appears that the content view lookup order documented at Content View Templates | Hugo doesn’t reflect real-life behavior.
Concretely, content views using the 1st pattern (/layouts/<TYPE>/<VIEW>.html) are always ignored. Only views under the 2nd pattern (/layouts/_default/<VIEW>.html) are rendered.
git clone --single-branch -b hugo-forum-topic-40414 https://github.com/jmooring/hugo-testing hugo-forum-topic-40414
cd hugo-forum-topic-40414
hugo server