diff -r 4dabf4db7cec -r b7072c97e0ca src/Pure/Build/build_job.scala
--- a/src/Pure/Build/build_job.scala	Wed Oct 08 10:49:18 2025 +0200
+++ b/src/Pure/Build/build_job.scala	Wed Oct 08 11:03:18 2025 +0200
@@ -357,7 +357,8 @@
           }
 
           session.runtime_statistics += Session.Consumer("ML_statistics") {
-            case Session.Runtime_Statistics(props) => runtime_statistics += props
+            case Session.Runtime_Statistics(props) =>
+              session.synchronized { runtime_statistics += props }
           }
 
           session.finished_theories += Session.Consumer[Document.Snapshot]("finished_theories") {