It appears this is a common problem as seen [here](https://github.com/rails/rails/issues/14401), [here](https://github.com/rails/rails/issues/36054) and [here](https://stackoverflow.com/questions/37507321/rails-namespacing-concerns-based-on-model-name). The solution is to move your namespaced concern out of the `concerns/` directory and under the `models` directory as per [this answer](https://stackoverflow.com/a/50889849/1299792). This is the same directory structure [they are using at Basecamp](https://twitter.com/dhh/status/964864438938492930)