Maybe it's a bug about the ngx_http_lua_init_worker function.

In the ngx_http_lua_init_worker function, use the http_ctx.main_conf = cycle->conf_ctx->main_conf to initialize. However, the value of main_conf may be modified in the later by merge_srv_conf or merge_loc_conf functions, which may cause some unexpected problems.

Forexample ngx_http_charset_merge_loc_conf function will be modify the value of mcf->recodes.

static char *
ngx_http_charset_merge_loc_conf(ngx_conf_t *cf, void *parent, void *child)
{
    ngx_http_charset_loc_conf_t *prev = parent;
    ngx_http_charset_loc_conf_t *conf = child;

    ngx_uint_t                     i;
    ngx_http_charset_recode_t     *recode;
    ngx_http_charset_main_conf_t  *mcf;

    if (ngx_http_merge_types(cf, &conf->types_keys, &conf->types,
                             &prev->types_keys, &prev->types,
                             ngx_http_charset_default_types)
        != NGX_OK)
    {
        return NGX_CONF_ERROR;
    }

    ngx_conf_merge_value(conf->override_charset, prev->override_charset, 0);
    ngx_conf_merge_value(conf->charset, prev->charset, NGX_HTTP_CHARSET_OFF);
    ngx_conf_merge_value(conf->source_charset, prev->source_charset,
                         NGX_HTTP_CHARSET_OFF);

    if (conf->charset == NGX_HTTP_CHARSET_OFF
        || conf->source_charset == NGX_HTTP_CHARSET_OFF
        || conf->charset == conf->source_charset)
    {
        return NGX_CONF_OK;
    }

    if (conf->source_charset >= NGX_HTTP_CHARSET_VAR
        || conf->charset >= NGX_HTTP_CHARSET_VAR)
    {
        return NGX_CONF_OK;
    }

    mcf = ngx_http_conf_get_module_main_conf(cf,
                                             ngx_http_charset_filter_module);
    recode = mcf->recodes.elts;
    for (i = 0; i < mcf->recodes.nelts; i++) {
        if (conf->source_charset == recode[i].src
            && conf->charset == recode[i].dst)
        {
            return NGX_CONF_OK;
        }
    }

    recode = ngx_array_push(&mcf->recodes);
    if (recode == NULL) {
        return NGX_CONF_ERROR;
    }

    recode->src = conf->source_charset;
    recode->dst = conf->charset;

    return NGX_CONF_OK;
}

So we should create of our own http_ctx.main_conf instead of directly reusing the current http_ctx.main_conf. Maybe it's fixed by PR , Plz check.

Start in src/ngx_http_lua_initworkerby.c and trace how ngx_http_lua_init_worker uses http_ctx.main_conf before merge_srv_conf and merge_loc_conf run. Compare this behavior with ngx_http_charset_merge_loc_conf in nginx's ngx_http_charset_filter_module.c and inspect the referenced commit. Done means determining whether the shared main configuration can be changed unexpectedly and whether the referenced commit resolves it.