I did this

$ nc -l -p 8080 -c "echo moom" &
[1] 28461
$ curl localhost:8080 -o dump
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (56) Recv failure: Connection reset by peer

The zeros in all columns is what's notable. The server sent 5 bytes. Adding -i doesn't help, the 5 bytes are not to be seen; they are not stored in the output file.

I expected the following

Not sure.

curl has support for "HTTP/0.9" which is header-less and is just a response-body until connection close. But since curl doesn't know what HTTP version that comes, it buffers the data to check the first header line. This small amount of data never gets checked since the code wants >5 bytes before it checks.

Since it never qualifies as a header, it should probably be considered a body and get delivered as such when it reaches the end of the transfer without confirming it to be a header.

curl/libcurl version

git master, but this is ancient bug. Presumably this is not an actual problem to many users.

operating system

doesn't matter