diff --git a/framework-platform/framework-platform.gradle b/framework-platform/framework-platform.gradle
index 713693887dff..4cb3cd55c61e 100644
--- a/framework-platform/framework-platform.gradle
+++ b/framework-platform/framework-platform.gradle
@@ -56,7 +56,7 @@ dependencies {
 		api("io.r2dbc:r2dbc-spi:1.0.0.RELEASE")
 		api("io.reactivex.rxjava3:rxjava:3.1.9")
 		api("io.smallrye.reactive:mutiny:1.10.0")
-		api("io.undertow:undertow-core:2.3.17.Final")
+		api("io.undertow:undertow-core:2.3.18.Final")
 		api("io.undertow:undertow-servlet:2.3.17.Final")
 		api("io.undertow:undertow-websockets-jsr:2.3.17.Final")
 		api("io.vavr:vavr:0.10.4")
diff --git a/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHttpHandlerAdapter.java b/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHttpHandlerAdapter.java
index 8c58eb159d8c..4517bc413247 100644
--- a/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHttpHandlerAdapter.java
+++ b/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHttpHandlerAdapter.java
@@ -66,25 +66,27 @@ public DataBufferFactory getDataBufferFactory() {
 
 	@Override
 	public void handleRequest(HttpServerExchange exchange) {
-		UndertowServerHttpRequest request = null;
-		try {
-			request = new UndertowServerHttpRequest(exchange, getDataBufferFactory());
-		}
-		catch (URISyntaxException ex) {
-			if (logger.isWarnEnabled()) {
-				logger.debug("Failed to get request URI: " + ex.getMessage());
+		exchange.dispatch(() -> {
+			UndertowServerHttpRequest request = null;
+			try {
+				request = new UndertowServerHttpRequest(exchange, getDataBufferFactory());
 			}
-			exchange.setStatusCode(400);
-			return;
-		}
-		ServerHttpResponse response = new UndertowServerHttpResponse(exchange, getDataBufferFactory(), request);
+			catch (URISyntaxException ex) {
+				if (logger.isWarnEnabled()) {
+					logger.debug("Failed to get request URI: " + ex.getMessage());
+				}
+				exchange.setStatusCode(400);
+				return;
+			}
+			ServerHttpResponse response = new UndertowServerHttpResponse(exchange, getDataBufferFactory(), request);
 
-		if (request.getMethod() == HttpMethod.HEAD) {
-			response = new HttpHeadResponseDecorator(response);
-		}
+			if (request.getMethod() == HttpMethod.HEAD) {
+				response = new HttpHeadResponseDecorator(response);
+			}
 
-		HandlerResultSubscriber resultSubscriber = new HandlerResultSubscriber(exchange, request);
-		this.httpHandler.handle(request, response).subscribe(resultSubscriber);
+			HandlerResultSubscriber resultSubscriber = new HandlerResultSubscriber(exchange, request);
+			this.httpHandler.handle(request, response).subscribe(resultSubscriber);
+		});
 	}
 
 
diff --git a/spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingMessageConversionIntegrationTests.java b/spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingMessageConversionIntegrationTests.java
index 582a086d9252..f5f2ff938f45 100644
--- a/spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingMessageConversionIntegrationTests.java
+++ b/spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingMessageConversionIntegrationTests.java
@@ -17,6 +17,7 @@
 package org.springframework.web.reactive.result.method.annotation;
 
 import java.nio.ByteBuffer;
+import java.time.Duration;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
@@ -331,6 +332,17 @@ void personTransformWithFlux(HttpServer httpServer) throws Exception {
 		assertThat(performPost("/person-transform/flux", JSON, req, JSON, PERSON_LIST).getBody()).isEqualTo(res);
 	}
 
+	@ParameterizedHttpServerTest // see gh-33885
+	void personTransformWithFluxDelayed(HttpServer httpServer) throws Exception {
+		startServer(httpServer);
+
+		List<?> req = asList(new Person("Robert"), new Person("Marie"));
+		List<?> res = asList(new Person("ROBERT"), new Person("MARIE"));
+		assertThat(performPost("/person-transform/flux-delayed", JSON, req, JSON, PERSON_LIST))
+				.satisfies(r -> assertThat(r.getBody()).isEqualTo(res))
+				.satisfies(r -> assertThat(r.getHeaders().getContentLength()).isNotZero());
+	}
+
 	@ParameterizedHttpServerTest
 	void personTransformWithObservable(HttpServer httpServer) throws Exception {
 		startServer(httpServer);
@@ -632,6 +644,11 @@ Flux<Person> transformFlux(@RequestBody Flux<Person> persons) {
 			return persons.map(person -> new Person(person.getName().toUpperCase()));
 		}
 
+		@PostMapping("/flux-delayed")
+		Flux<Person> transformDelayed(@RequestBody Flux<Person> persons) {
+			return transformFlux(persons).delayElements(Duration.ofMillis(10));
+		}
+
 		@PostMapping("/observable")
 		Observable<Person> transformObservable(@RequestBody Observable<Person> persons) {
 			return persons.map(person -> new Person(person.getName().toUpperCase()));