Search.log in Search job inspector shows that searches were only dispatched to the 2 existing cluster indexers but not to the new indexers.
The search GUI shows the field name, splunkservergroup field shows that dmc_xxx groups. This means the deployment monitoring console, DMC is enabled on the search head. With DMC enabled on SH, only search peers that are members of the indexer group are searched by default. Because all searches that run on the monitoring console instance follow this behavior, non-monitoring console searches might have incomplete results.
"Do Not Use Production Search Head for DMC" which is also documented in the link below - Resolution: Switching to "standalone" next to "distributed" fixed the issue.