I’m having some weird issue when people make requests with many cookies:

2020-09-17 19:12:50.285 [error] Cowboy returned 431 because it was unable to parse the request headers.This may happen because there are no headers, or there are too many headers
or the header name or value are too large (such as a large cookie).

More specific reason is:

    :"A header value is larger than configuration allows. (RFC7230 3.2.5, RFC6585 5)"

You can customize those limits when configuring your http/https server. The configuration option and default values are shown below:

    protocol_options: [
      max_header_name_length: 64,
      max_header_value_length: 4096,
      max_headers: 100
    ]

so I added this bit to my config/config.exs:

config :horse_racing_nation_web, HorseRacingNationWeb.News.Endpoint,
  http: [protocol_options: [max_header_value_length: 8192]],
  ...

…which fixes it in development, but when I build a release I still get the 431.

I can see the option makes it through to the sys.config in the release:

   {'Elixir.HorseRacingNationWeb.News.Endpoint',
       [{http,
            [{protocol_options,
                 [{max_header_value_length,8192}]}]},

Any ideas why it works in development but not production?