diff --git a/src/NHibernate.Test/NHSpecificTest/GH3607/FixtureByCode.cs b/src/NHibernate.Test/NHSpecificTest/GH3607/FixtureByCode.cs
new file mode 100644
index 00000000000..0ce9b1a592c
--- /dev/null
+++ b/src/NHibernate.Test/NHSpecificTest/GH3607/FixtureByCode.cs
@@ -0,0 +1,31 @@
+using NHibernate.Cfg;
+using NHibernate.Cfg.MappingSchema;
+using NHibernate.Mapping.ByCode;
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH3607
+{
+	/// <summary>
+	/// By code mapping serialization failure since v5.4.1. Adapted from <see href="https://github.com/craigfowler/NHibernate.XmlConversionBug" />.
+	/// </summary>
+	[TestFixture]
+	public class FixtureByCode : TestCaseMappingByCode
+	{
+		protected override HbmMapping GetMappings()
+		{
+			var mapper = new ModelMapper();
+			mapper.AddMappings(new[] { typeof(OrderMapping), typeof(LineItemMapping), typeof(LineItemDataMapping) });
+			return mapper.CompileMappingForAllExplicitlyAddedEntities();
+		}
+
+		[Test]
+		public void SerializeMappingToXml()
+		{
+			var mapping = GetMappings();
+			string serialized = "";
+			Assert.That(() => serialized = mapping.AsString(), Throws.Nothing, "Mapping serialization failure");
+			var config = new Configuration();
+			Assert.That(() => config.AddXml(serialized), Throws.Nothing, "Configuration with serialized mapping has failed");
+		}
+	}
+}
diff --git a/src/NHibernate.Test/NHSpecificTest/GH3607/LineItem.cs b/src/NHibernate.Test/NHSpecificTest/GH3607/LineItem.cs
new file mode 100644
index 00000000000..146a7407b26
--- /dev/null
+++ b/src/NHibernate.Test/NHSpecificTest/GH3607/LineItem.cs
@@ -0,0 +1,15 @@
+namespace NHibernate.Test.NHSpecificTest.GH3607
+{
+	public class LineItem
+	{
+		public virtual int Id { get; set; }
+
+		public virtual Order ParentOrder { get; set; }
+
+		public virtual string ItemName { get; set; }
+
+		public virtual decimal Amount { get; set; }
+
+		public virtual LineItemData Data { get; set; }
+	}
+}
diff --git a/src/NHibernate.Test/NHSpecificTest/GH3607/LineItemData.cs b/src/NHibernate.Test/NHSpecificTest/GH3607/LineItemData.cs
new file mode 100644
index 00000000000..5bcfa3ad64c
--- /dev/null
+++ b/src/NHibernate.Test/NHSpecificTest/GH3607/LineItemData.cs
@@ -0,0 +1,9 @@
+namespace NHibernate.Test.NHSpecificTest.GH3607
+{
+	public class LineItemData
+	{
+		public virtual LineItem LineItem { get; set; }
+
+		public virtual string Data { get; set; }
+	}
+}
diff --git a/src/NHibernate.Test/NHSpecificTest/GH3607/LineItemDataMapping.cs b/src/NHibernate.Test/NHSpecificTest/GH3607/LineItemDataMapping.cs
new file mode 100644
index 00000000000..66634421a3b
--- /dev/null
+++ b/src/NHibernate.Test/NHSpecificTest/GH3607/LineItemDataMapping.cs
@@ -0,0 +1,14 @@
+using NHibernate.Mapping.ByCode.Conformist;
+
+namespace NHibernate.Test.NHSpecificTest.GH3607
+{
+	public class LineItemDataMapping : ClassMapping<LineItemData>
+	{
+		public LineItemDataMapping()
+		{
+			OneToOne(x => x.LineItem, m => m.Constrained(true));
+
+			Property(x => x.Data);
+		}
+	}
+}
diff --git a/src/NHibernate.Test/NHSpecificTest/GH3607/LineItemMapping.cs b/src/NHibernate.Test/NHSpecificTest/GH3607/LineItemMapping.cs
new file mode 100644
index 00000000000..3918b0a84f3
--- /dev/null
+++ b/src/NHibernate.Test/NHSpecificTest/GH3607/LineItemMapping.cs
@@ -0,0 +1,21 @@
+using NHibernate.Mapping.ByCode;
+using NHibernate.Mapping.ByCode.Conformist;
+
+namespace NHibernate.Test.NHSpecificTest.GH3607
+{
+	public class LineItemMapping : ClassMapping<LineItem>
+	{
+		public LineItemMapping()
+		{
+			Id(x => x.Id, m => m.Generator(new IdentityGeneratorDef()));
+
+			Property(x => x.ItemName);
+
+			Property(x => x.Amount);
+
+			ManyToOne(x => x.ParentOrder);
+
+			ManyToOne(x => x.Data);
+		}
+	}
+}
diff --git a/src/NHibernate.Test/NHSpecificTest/GH3607/Order.cs b/src/NHibernate.Test/NHSpecificTest/GH3607/Order.cs
new file mode 100644
index 00000000000..be3b9d05ffc
--- /dev/null
+++ b/src/NHibernate.Test/NHSpecificTest/GH3607/Order.cs
@@ -0,0 +1,14 @@
+using System;
+using System.Collections.Generic;
+
+namespace NHibernate.Test.NHSpecificTest.GH3607
+{
+	public class Order
+	{
+		public virtual int Id { get; set; }
+
+		public virtual DateTime CreatedDate { get; set; }
+
+		public virtual ISet<LineItem> Items { get; protected set; } = new HashSet<LineItem>();
+	}
+}
diff --git a/src/NHibernate.Test/NHSpecificTest/GH3607/OrderMapping.cs b/src/NHibernate.Test/NHSpecificTest/GH3607/OrderMapping.cs
new file mode 100644
index 00000000000..036c65e47a5
--- /dev/null
+++ b/src/NHibernate.Test/NHSpecificTest/GH3607/OrderMapping.cs
@@ -0,0 +1,22 @@
+using NHibernate.Mapping.ByCode;
+using NHibernate.Mapping.ByCode.Conformist;
+
+namespace NHibernate.Test.NHSpecificTest.GH3607
+{
+	public class OrderMapping : ClassMapping<Order>
+	{
+		public OrderMapping()
+		{
+			Table("`Order`");
+			Id(x => x.Id, m => m.Generator(new IdentityGeneratorDef()));
+
+			Property(x => x.CreatedDate);
+
+			Set(x => x.Items, m =>
+			{
+				m.Inverse(true);
+				m.OptimisticLock(true);
+			}, a => a.OneToMany());
+		}
+	}
+}
diff --git a/src/NHibernate/Cfg/MappingSchema/HbmOneToOne.cs b/src/NHibernate/Cfg/MappingSchema/HbmOneToOne.cs
index e0740e17937..aec82a44ee4 100644
--- a/src/NHibernate/Cfg/MappingSchema/HbmOneToOne.cs
+++ b/src/NHibernate/Cfg/MappingSchema/HbmOneToOne.cs
@@ -18,9 +18,13 @@ public string Access
 			get { return access; }
 		}
 
+		// 6.0 Todo : remove XmlIgnore after removing the setter. See #3607 fix.
+		[XmlIgnore]
 		public bool OptimisticLock
 		{
 			get => optimisticlock;
+			// Since v5.4.10
+			[Obsolete("Providing a setter for OptimisticLock was unintended and will be removed.")]
 			set => optimisticlock = value;
 		}