diff --git a/core/src/test/java/ai/timefold/solver/core/testutil/AbstractMeterTest.java b/core/src/test/java/ai/timefold/solver/core/testutil/AbstractMeterTest.java
index 9633537bf43..fde20f05f3f 100644
--- a/core/src/test/java/ai/timefold/solver/core/testutil/AbstractMeterTest.java
+++ b/core/src/test/java/ai/timefold/solver/core/testutil/AbstractMeterTest.java
@@ -8,7 +8,8 @@
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
-import org.junit.jupiter.api.parallel.ResourceLock;
+import org.junit.jupiter.api.parallel.Execution;
+import org.junit.jupiter.api.parallel.ExecutionMode;
 
 import io.micrometer.core.instrument.Metrics;
 import io.micrometer.core.instrument.MockClock;
@@ -16,7 +17,8 @@
 import io.micrometer.core.instrument.simple.SimpleConfig;
 import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
 
-@ResourceLock(value = "metrics") // Mutual exclusion to prevent flaky tests.
+// We also want to ensure that tests of the same suite do not update the registry concurrently
+@Execution(ExecutionMode.SAME_THREAD)
 public abstract class AbstractMeterTest {
 
     @BeforeEach // To guard against nasty tests which do not do this.
diff --git a/core/src/test/java/ai/timefold/solver/core/testutil/PlannerTestUtils.java b/core/src/test/java/ai/timefold/solver/core/testutil/PlannerTestUtils.java
index 3390a62e0de..a6ca194a416 100644
--- a/core/src/test/java/ai/timefold/solver/core/testutil/PlannerTestUtils.java
+++ b/core/src/test/java/ai/timefold/solver/core/testutil/PlannerTestUtils.java
@@ -70,11 +70,16 @@ public static <Solution_> SolverConfig buildSolverConfig(Class<Solution_> soluti
                                 new TerminationConfig().withStepCountLimit(TERMINATION_STEP_COUNT_LIMIT)));
     }
 
-    public static <Solution_> Solution_ solve(SolverConfig solverConfig, Solution_ problem) {
+    public static synchronized <Solution_> Solution_ solve(SolverConfig solverConfig, Solution_ problem) {
         return solve(solverConfig, problem, true);
     }
 
-    public static <Solution_> Solution_ solve(SolverConfig solverConfig, Solution_ problem, boolean bestSolutionEventExists) {
+    /**
+     * We need to synchronize the solving process because there are some shared resources, such as the Meter registry,
+     * which could be updated concurrently.
+     */
+    public static synchronized <Solution_> Solution_ solve(SolverConfig solverConfig, Solution_ problem,
+            boolean bestSolutionEventExists) {
         SolverFactory<Solution_> solverFactory = SolverFactory.create(solverConfig);
         var solver = solverFactory.buildSolver();
         var eventBestSolutionRef = new AtomicReference<Solution_>();