ngx.exit(415) during read() loop does *not* cleanly exit - binary data spewed in log files

[braider]

            local upname = nil
            while true do
                local typ, res, err = form:read()
-- bla bla do something
                
                if typ == "header" then
                    for k,v in pairs(res) do
                        local ext = string.match(v,'.+(%.7z.*)"') -- say we only want to permit files with this extension
                        if ext then
                            upname = ext
                        end
                    end
                end

               -- crude but we found our extension, if we didn't then 
                if not upname then
                    -- cleanup()
                    ngx.status = 415
                    ngx.exit(415) -- makes nginx dump binary fuzz into the error.log 
                end

-- sample bad output in error.log  after connnection RST issued

09:17:16 [info] 6461#6461: *511 client sent invalid method while readin                                                                 g client request line, client: 127.0.0.1, server: xxxxx, request: ▒▒"▒▒  ▒▒f

^^^ that kind of rubbish dumped in the error.log

HOWEVER - the Correct behaviour occurs if the file upload is small (<300-400k) - presumably all read into a single buffer.. by correct i mean the upload is terminated with a redirect to the configured error_page 415 /415.html and the content of the error page is duly served.

If the file is larger than that (500k+) ngx.exit(415) totally fails and spews bad output into error.log

What's going on here? Pipes not emptying properly? Request line disappearing? This is easily reproducible on the latest openresty+debian buster