custom plugin for clear some specific request headers, but upstream maintains these parameters

[lltime]

apisix version : 2.6

plugin code :

function _M.header_filter(conf, ctx)

    if not conf.clear_keys then
        return
    end

    local field_cnt = #conf.clear_keys

    for i = 1, field_cnt do
        local value = core.request.header(ctx, conf.clear_keys[i])
        if value then 
            ngx.req.set_header(conf.clear_keys[i], nil)
            -- ngx.req.clear_header(conf.clear_keys[i])
            core.log.warn("clear header key = ", conf.clear_keys[i], 
                          " , value = ", value)
        end
    end
end 

log print

2021/08/31 21:51:39 [warn] 7642#7642: *1863 [lua] global-clear-header.lua:69: phase_func(): clear header key = das-clear , value = 123123123123123 while reading response header from upstream, client: 219.140.141.64, server: _, request: "GET /am/v1/api/users/current HTTP/1.1", upstream: "http://192.168.0.245:8802/v1/api/users/current", host: "xxxxx.com"

postman

I don't know why this happened. Thank you for taking the time to help solve the problem.