diff --git a/extensions/resteasy-reactive/quarkus-resteasy-reactive/deployment/src/test/java/io/quarkus/resteasy/reactive/server/test/mediatype/ContentTypeCaseTest.java b/extensions/resteasy-reactive/quarkus-resteasy-reactive/deployment/src/test/java/io/quarkus/resteasy/reactive/server/test/mediatype/ContentTypeCaseTest.java
new file mode 100644
index 0000000000000..7698788d50dc9
--- /dev/null
+++ b/extensions/resteasy-reactive/quarkus-resteasy-reactive/deployment/src/test/java/io/quarkus/resteasy/reactive/server/test/mediatype/ContentTypeCaseTest.java
@@ -0,0 +1,63 @@
+package io.quarkus.resteasy.reactive.server.test.mediatype;
+
+import static io.restassured.RestAssured.given;
+import static org.hamcrest.Matchers.is;
+
+import java.util.function.Supplier;
+
+import javax.ws.rs.GET;
+import javax.ws.rs.HeaderParam;
+import javax.ws.rs.Path;
+import javax.ws.rs.core.MediaType;
+import javax.ws.rs.core.Response;
+
+import org.jboss.shrinkwrap.api.ShrinkWrap;
+import org.jboss.shrinkwrap.api.spec.JavaArchive;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+import io.quarkus.test.QuarkusUnitTest;
+
+public class ContentTypeCaseTest {
+
+    @RegisterExtension
+    static QuarkusUnitTest test = new QuarkusUnitTest()
+            .setArchiveProducer(new Supplier<>() {
+                @Override
+                public JavaArchive get() {
+                    return ShrinkWrap.create(JavaArchive.class)
+                            .addClasses(HelloResource.class);
+                }
+            });
+
+    @Test
+    public void test() {
+        given().header("test", "TeXt/Plain").get("/hello")
+                .then()
+                .statusCode(200)
+                .contentType("text/plain")
+                .body(is("text/plain"));
+
+        given().header("test", "text/plain").get("/hello")
+                .then()
+                .statusCode(200)
+                .contentType("text/plain")
+                .body(is("text/plain"));
+
+        given().header("test", "TEXT/PLAIN").get("/hello")
+                .then()
+                .statusCode(200)
+                .contentType("text/plain")
+                .body(is("text/plain"));
+    }
+
+    @Path("hello")
+    public static class HelloResource {
+
+        @GET
+        public Response hello(@HeaderParam("test") String contentType) {
+            MediaType mediaType = MediaType.valueOf(contentType);
+            return Response.ok(mediaType.toString()).header("content-type", mediaType).build();
+        }
+    }
+}
diff --git a/independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/headers/MediaTypeHeaderDelegate.java b/independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/headers/MediaTypeHeaderDelegate.java
index 9a450ad606f4a..cbe8b2fb02cce 100644
--- a/independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/headers/MediaTypeHeaderDelegate.java
+++ b/independent-projects/resteasy-reactive/common/runtime/src/main/java/org/jboss/resteasy/reactive/common/headers/MediaTypeHeaderDelegate.java
@@ -14,8 +14,8 @@ public class MediaTypeHeaderDelegate implements RuntimeDelegate.HeaderDelegate<M
     public static final MediaTypeHeaderDelegate INSTANCE = new MediaTypeHeaderDelegate();
     private static final int MAX_MT_CACHE_SIZE = 200;
     private static final char[] quotedChars = "()<>@,;:\\\"/[]?= \t\r\n".toCharArray();
-    private static final Map<String, MediaType> map = new ConcurrentHashMap<String, MediaType>();
-    private static final Map<MediaType, String> reverseMap = new ConcurrentHashMap<MediaType, String>();
+    private static final Map<String, MediaType> map = new ConcurrentHashMap<>();
+    private static final Map<MediaType, String> reverseMap = new ConcurrentHashMap<>();
 
     protected static boolean isValid(String str) {
         if (str == null || str.length() == 0)
@@ -55,7 +55,7 @@ public static MediaType parse(String type) {
                 reverseMap.clear();
             }
             map.put(type, result);
-            reverseMap.put(result, type);
+            reverseMap.put(result, internalToString(result));
         }
         return result;
     }
@@ -141,10 +141,10 @@ public String toString(MediaType o) {
         return result;
     }
 
-    private String internalToString(MediaType type) {
-        StringBuilder buf = new StringBuilder();
+    private static String internalToString(MediaType type) {
+        StringBuilder buf = new StringBuilder(type.getType().length() + type.getSubtype().length() + 1);
 
-        buf.append(type.getType().toLowerCase()).append("/").append(type.getSubtype().toLowerCase());
+        buf.append(type.getType().toLowerCase()).append('/').append(type.getSubtype().toLowerCase());
         if (type.getParameters() == null || type.getParameters().size() == 0)
             return buf.toString();
         for (String name : type.getParameters().keySet()) {