This is a bug in Req, we should be calling your request steps when making the redirect. I’d have to track the history how we ended up there but FWIW here’s a minimal reproduction:

Mix.install([
  {:req, path: "~/src/req"},
  :bandit
])

{:ok, _} =
  Bandit.start_link(
    plug: fn
      conn, _ when conn.request_path == "/" ->
        conn
        |> Plug.Conn.put_resp_header("location", "/foo bar")
        |> Plug.Conn.send_resp(301, "redirecting")

      conn, _ when conn.request_path == "/foo%20bar" ->
        Plug.Conn.send_resp(conn, 200, "ok")
    end,
    port: 8000
  )

Req.new()
|> Req.Request.prepend_request_steps(
  encode_url: fn r ->
    dbg(:encode_url)
    update_in(r.url.path, &(&1 && URI.encode(&1)))
  end
)
|> Req.get!(url: "http://localhost:8000")

and here’s a fix:

diff --git a/lib/req/steps.ex b/lib/req/steps.ex
index 33e332e..8e8fc73 100644
--- a/lib/req/steps.ex
+++ b/lib/req/steps.ex
@@ -1813,6 +1813,8 @@ defmodule Req.Steps do
           |> build_redirect_request(response, location)
           |> Req.Request.put_private(:req_redirect_count, redirect_count + 1)

+        request = put_in(request.current_request_steps, Keyword.keys(request.request_steps))
+
         {request, response_or_exception} = Req.Request.run_request(request)
         Req.Request.halt(request, response_or_exception)
       else

Stay tuned!