Upstream defaults to the bundled JDK, which the ebuild removes.

--- a/bin/elasticsearch-env
+++ b/bin/elasticsearch-env
@@ -32,6 +32,17 @@
 done
 ES_HOME=`dirname "$ES_HOME"`
 
+# Select a JDK via java-config unless ES_JAVA_HOME is set
+if [ -z "$ES_JAVA_HOME" ]; then
+  GENTOO_VM=$(depend-java-query --get-vm virtual/jdk:21)
+  if [ -n "$GENTOO_VM" ]; then
+    ES_JAVA_HOME=$(java-config-2 --select-vm="$GENTOO_VM" --jdk-home)
+  else
+    echo "no Java 21 VM found. Install virtual/jdk:21 or set ES_JAVA_HOME" >&2
+    exit 1
+  fi
+fi
+
 # now set the path to java
 if [ ! -z "$ES_JAVA_HOME" ]; then
   JAVA="$ES_JAVA_HOME/bin/java"