Is "enum x : UInt32" cast to "enum x : int" when serializing?

This would make sense as I would imagine Unity stores all enums as though they are flags, with the implicit 'Everything' field always set to -1. The SerializedProperty only has value properties for intValue and longValue as well - and according to the docs, uint values will be set via the intValue field (and therefore implicitly cast to int32 for some reason). To get around this, you can make a custom PropertyAttribute and PropertyDrawer that uses 'property.longValue' instead (as long as you don't need ulong);

 using System;
 using UnityEngine;
 
 public class UIntEnumAttribute : PropertyAttribute
 {
     public Type type;
     public bool flags;
 
     public UIntEnumAttribute (Type a_Type, bool a_Flags = false)
     {
         type = a_Type;
         flags = a_Flags;
     }
 }

PropertyDrawer (in an Editor folder):

 using System;
 using UnityEngine;
 using UnityEditor;
 
 [CustomPropertyDrawer (typeof (UIntEnumAttribute))]
 public class UIntEnumDrawer : PropertyDrawer
 {
     public UIntEnumAttribute EnumAttribute => (UIntEnumAttribute)attribute;
 
     public override void OnGUI (Rect position, SerializedProperty property, GUIContent label)
     {
         Enum value = (Enum)Enum.ToObject (EnumAttribute.type, (uint)property.longValue);
         if (EnumAttribute.flags)
         {
             value = EditorGUI.EnumFlagsField (position, label, value);
         }
         else
         {
             value = EditorGUI.EnumPopup (position, label, value);
         }
 
         property.longValue = (uint)Enum.ToObject (EnumAttribute.type, value);
     }
 }

Usage example:

 public enum MessageId : uint
 {
     StartPingCheck = 0x00000001,
     CompletePingCheck = 0x00000002,
     AgentUpdate = 0x00000004,
     CoarseLocationUpdate = 0x0000ff06,
     AttachedSound = 0x0000ff0d,
     Wrapper = 0xffff0001,
     UseCircuitCode = 0xffff0003
 }
 
 [SerializeField, UIntEnum (typeof (MessageId))] private MessageId m_MessageID = MessageId.StartPingCheck;