diff --git a/test/framework/src/main/java/org/elasticsearch/test/MockLogAppender.java b/test/framework/src/main/java/org/elasticsearch/test/MockLogAppender.java
index ccb010e2a915e..22a12e74c3516 100644
--- a/test/framework/src/main/java/org/elasticsearch/test/MockLogAppender.java
+++ b/test/framework/src/main/java/org/elasticsearch/test/MockLogAppender.java
@@ -24,8 +24,8 @@
 import org.apache.logging.log4j.core.filter.RegexFilter;
 import org.elasticsearch.common.regex.Regex;
 
-import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.regex.Pattern;
 
 import static org.hamcrest.CoreMatchers.equalTo;
@@ -42,7 +42,12 @@ public class MockLogAppender extends AbstractAppender {
 
     public MockLogAppender() throws IllegalAccessException {
         super("mock", RegexFilter.createFilter(".*(\n.*)*", new String[0], false, null, null), null);
-        expectations = new ArrayList<>();
+        /*
+         * We use a copy-on-write array list since log messages could be appended while we are setting up expectations. When that occurs,
+         * we would run into a concurrent modification exception from the iteration over the expectations in #append, concurrent with a
+         * modification from #addExpectation.
+         */
+        expectations = new CopyOnWriteArrayList<>();
     }
 
     public void addExpectation(LoggingExpectation expectation) {
diff --git a/test/framework/src/main/java/org/elasticsearch/transport/AbstractSimpleTransportTestCase.java b/test/framework/src/main/java/org/elasticsearch/transport/AbstractSimpleTransportTestCase.java
index 3fddc27297b1d..960d25c960946 100644
--- a/test/framework/src/main/java/org/elasticsearch/transport/AbstractSimpleTransportTestCase.java
+++ b/test/framework/src/main/java/org/elasticsearch/transport/AbstractSimpleTransportTestCase.java
@@ -993,7 +993,6 @@ public void handleException(TransportException exp) {
     }
 
     @TestLogging(value = "org.elasticsearch.transport.TransportService.tracer:trace")
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/40586")
     public void testTracerLog() throws Exception {
         TransportRequestHandler<TransportRequest> handler = (request, channel, task) -> channel.sendResponse(new StringMessageResponse(""));
         TransportRequestHandler<StringMessageRequest> handlerWithError = (request, channel, task) -> {