Address comments

src/rdkafka_cgrp.c

@@ -2333,6 +2333,10 @@ static int rd_kafka_cgrp_metadata_refresh(rd_kafka_cgrp_t *rkcg,
 
         rd_list_init(&topics, 8, rd_free);
 
+        /* Insert all non-wildcard topics in cache. */
+        rd_kafka_metadata_cache_hint_rktparlist(
+            rkcg->rkcg_rk, rkcg->rkcg_subscription, NULL, 0 /*dont replace*/);
+
         if (rkcg->rkcg_flags & RD_KAFKA_CGRP_F_WILDCARD_SUBSCRIPTION) {
                 /* For wildcard subscriptions make sure the
                  * cached full metadata isn't too old. */
@@ -5007,8 +5011,9 @@ rd_kafka_cgrp_subscription_set(rd_kafka_cgrp_t *rkcg,
         rkcg->rkcg_subscription = rktparlist;
         if (rkcg->rkcg_subscription) {
                 /* Insert all non-wildcard topics in cache immediately.
-                 * to avoid problems with subsequent metadata
-                 * requests. */
+                 * Otherwise a manual full metadata request could
+                 * not cache the hinted topic and return an
+                 * UNKNOWN_TOPIC_OR_PART error to the user. See #4589. */
                 rd_kafka_metadata_cache_hint_rktparlist(
                     rkcg->rkcg_rk, rkcg->rkcg_subscription, NULL,
                     0 /*dont replace*/);