diff --git a/tooling/metamodel-generator/src/main/java/org/hibernate/processor/util/StringUtil.java b/tooling/metamodel-generator/src/main/java/org/hibernate/processor/util/StringUtil.java
index 6fd474b51cdb..5b64904275d1 100644
--- a/tooling/metamodel-generator/src/main/java/org/hibernate/processor/util/StringUtil.java
+++ b/tooling/metamodel-generator/src/main/java/org/hibernate/processor/util/StringUtil.java
@@ -98,18 +98,21 @@ public static String nameToMethodName(String name) {
 	}
 
 	public static String getUpperUnderscoreCaseFromLowerCamelCase(String lowerCamelCaseString) {
-		if ( lowerCamelCaseString.length() == 1 && isUpperCase( lowerCamelCaseString.charAt( 0 ) ) ) {
-			return "_" + lowerCamelCaseString;
-		}
 		final StringBuilder result = new StringBuilder();
 		int position = 0;
+		boolean wasLowerCase = false;
 		while ( position < lowerCamelCaseString.length() ) {
 			final int codePoint = lowerCamelCaseString.codePointAt( position );
-			if ( position>0 && isUpperCase( codePoint ) ) {
+			final boolean isUpperCase = isUpperCase( codePoint );
+			if ( wasLowerCase && isUpperCase ) {
 				result.append('_');
 			}
 			result.appendCodePoint( toUpperCase( codePoint ) );
 			position += charCount( codePoint );
+			wasLowerCase = !isUpperCase;
+		}
+		if ( result.toString().equals( lowerCamelCaseString ) ) {
+			result.insert(0, '_');
 		}
 		return result.toString();
 	}
diff --git a/tooling/metamodel-generator/src/main/java/org/hibernate/processor/util/TypeUtils.java b/tooling/metamodel-generator/src/main/java/org/hibernate/processor/util/TypeUtils.java
index 33afa615b092..822c8d129faa 100644
--- a/tooling/metamodel-generator/src/main/java/org/hibernate/processor/util/TypeUtils.java
+++ b/tooling/metamodel-generator/src/main/java/org/hibernate/processor/util/TypeUtils.java
@@ -36,7 +36,6 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
-import java.util.function.Function;
 
 import static java.beans.Introspector.decapitalize;
 import static org.hibernate.processor.util.AccessTypeInformation.DEFAULT_ACCESS_TYPE;
@@ -190,12 +189,7 @@ public static String extractClosestRealTypeAsString(TypeMirror type, Context con
 				return context.getTypeUtils().getDeclaredType(
 						typeElement,
 						declaredType.getTypeArguments().stream()
-								.map( new Function<TypeMirror, TypeMirror>() {
-											@Override
-											public @Nullable TypeMirror apply(TypeMirror arg) {
-												return extractClosestRealType( arg, context, beingVisited );
-											}
-										} )
+								.map( arg -> extractClosestRealType( arg, context, beingVisited ) )
 								.toArray( TypeMirror[]::new )
 				);
 			default:
diff --git a/tooling/metamodel-generator/src/test/java/org/hibernate/processor/test/uppercase/Person.java b/tooling/metamodel-generator/src/test/java/org/hibernate/processor/test/uppercase/Person.java
new file mode 100644
index 000000000000..078e9e4b1b77
--- /dev/null
+++ b/tooling/metamodel-generator/src/test/java/org/hibernate/processor/test/uppercase/Person.java
@@ -0,0 +1,14 @@
+/*
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ * Copyright Red Hat Inc. and Hibernate Authors
+ */
+package org.hibernate.processor.test.uppercase;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.Id;
+
+@Entity
+public class Person {
+	@Id String SSN;
+	String UserID;
+}
diff --git a/tooling/metamodel-generator/src/test/java/org/hibernate/processor/test/uppercase/UppercaseTest.java b/tooling/metamodel-generator/src/test/java/org/hibernate/processor/test/uppercase/UppercaseTest.java
new file mode 100644
index 000000000000..d765288a8246
--- /dev/null
+++ b/tooling/metamodel-generator/src/test/java/org/hibernate/processor/test/uppercase/UppercaseTest.java
@@ -0,0 +1,29 @@
+/*
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ * Copyright Red Hat Inc. and Hibernate Authors
+ */
+package org.hibernate.processor.test.uppercase;
+
+import org.hibernate.processor.test.util.CompilationTest;
+import org.hibernate.processor.test.util.WithClasses;
+import org.junit.Test;
+
+import static org.hibernate.processor.test.util.TestUtil.assertMetamodelClassGeneratedFor;
+import static org.hibernate.processor.test.util.TestUtil.assertPresenceOfFieldInMetamodelFor;
+import static org.hibernate.processor.test.util.TestUtil.getMetaModelSourceAsString;
+
+public class UppercaseTest extends CompilationTest {
+
+	@Test
+	@WithClasses(value = Person.class)
+	public void test() {
+		System.out.println( getMetaModelSourceAsString( Person.class ) );
+
+		assertMetamodelClassGeneratedFor( Person.class );
+
+		assertPresenceOfFieldInMetamodelFor( Person.class, "SSN" );
+		assertPresenceOfFieldInMetamodelFor( Person.class, "_SSN" );
+		assertPresenceOfFieldInMetamodelFor( Person.class, "UserID" );
+		assertPresenceOfFieldInMetamodelFor( Person.class, "USER_ID" );
+	}
+}