SOLR-12739 made autoscaling as the default replica placement strategy. However in the absence of SOLR-12845, replicas can be placed without any regards for maxShardsPerNode causing multiple replicas of the same shard to be placed on the same node together. Also it was reported in SOLR-13247 that createNodeSet is not being respected as well. SOLR-13159 was an early signal of the problem but it was not reproducible and there was a DNS problem in the cluster too so the root cause was not clear then.
I am creating this blocker issue because as it stands today, we cannot guarantee the layout of new collections. At a minimum, we should revert to using the legacy replica assignment policy or add default policies with
SOLR-12845 and have createNodeSet work. Related but not mandatory would be to fix SOLR-12847 as well.