This is a regression from Rails 3.x, but it could be by design: ActionDispatch::ParamsParser#parse_formatted_parameters will read the input via ActionDispatch::Request, and not rewind it.
It broke Grape mounted on Rails, see intridea/grape#417.
I think it might make sense to preserve input data in this case so that other middleware or consumers can read it. At least in Rails 3.2 this wasn't an issue since this data wasn't read.
This looks to have been fixed in f6746c0, as covered by rails/rails#11345. Can you confirm and close?
RIght. Thank you for reporting.