ngx.resp.get_headers(): Why is the response header incomplete?

config:
location =/version {
default_type 'text/plain';

        content_by_lua_file /usr/local/lmrs/lua/api/version.lua;
    
        log_by_lua '
	ngx.log(ngx.ERR,"header: ", require"cjson".encode(ngx.resp.get_headers()))
    ';
    }

request:
curl -i "http://127.0.0.1:8088/version"
HTTP/1.1 200 OK
Server: nginx/1.12.2
Date: Tue, 10 Sep 2019 13:17:46 GMT
Content-Type: text/plain
Transfer-Encoding: chunked
Connection: keep-alive

log:
2019/09/10 09:12:14 [error] 123144#0: *3 [lua] log_by_lua(lmrs.conf:195):2: header: {"transfer-encoding":"chunked","content-type":"text/plain","connection":"close"} while logging request, client: 127.0.0.1, server: , request: "GET /version HTTP/1.1", host: "127.0.0.1:8088"

Why is the response header incomplete?

Start from the ngx.resp.get_headers entry point and reproduce the reported configuration with the curl request. Compare the response headers with the log output and determine whether the missing headers reflect expected request-phase behavior or a defect; document the explanation or required change.