Fix double-encoding of non-string JSON values in nested blocks and add SingleBlock mapper

uSync.Core/Extensions/JsonTextExtensions.cs

@@ -520,4 +520,15 @@ public static bool IsJsonEqual(this object currentObject, object newObject)
 
     #endregion
 
+    #region Type Checks 
+
+    /// <summary>
+    ///  checks if the value is a non-string JSON value (array, object, number, boolean).
+    /// </summary>
+    public static bool IsNonStringJsonValue(this object? value)
+        => value is JsonElement { ValueKind: not JsonValueKind.String and not JsonValueKind.Undefined }
+           || value is JsonArray or JsonObject;
+
+    #endregion
+
 }

uSync.Core/Mapping/Mappers/SingleBlockMapper.cs

@@ -0,0 +1,23 @@
+using Microsoft.Extensions.Logging;
+
+using Umbraco.Cms.Core;
+using Umbraco.Cms.Core.Models.Blocks;
+using Umbraco.Cms.Core.Services;
+
+namespace uSync.Core.Mapping.Mappers;
+
+internal class SingleBlockMapper : SyncBlockMapperBase<SingleBlockValue>, ISyncMapper
+{
+    public override string Name => "NuBlock Single Block mapper";
+
+    public override string[] Editors => [Constants.PropertyEditors.Aliases.SingleBlock];
+
+    public SingleBlockMapper(
+        IEntityService entityService,
+        IContentTypeService contentTypeService,
+        Lazy<SyncValueMapperCollection> mapperCollection,
+        ILogger<SingleBlockMapper> logger)
+        : base(entityService, contentTypeService, mapperCollection, logger)
+    {
+    }
+}

uSync.Core/Mapping/SyncBlockMapperBase.cs

@@ -1,6 +1,7 @@
 using Microsoft.Extensions.Logging;
 
 using System.Collections;
+using System.Text.Json;
 using System.Text.Json.Nodes;
 
 using Umbraco.Cms.Core;
@@ -61,7 +62,17 @@ public SyncBlockMapperBase(
             _logger.LogDebug("Importing block value for {PropertyEditorAlias} {valueType}", propertyType.PropertyEditorAlias, value?.GetType().Name ?? "blank");
 
         var importString = SyncBlockMapperBase<TBlockValue>.GetStringValue(value) ?? string.Empty;
-        return await _mapperCollection.Value.GetImportValueAsync(importString, propertyType, options);
+        var result = await _mapperCollection.Value.GetImportValueAsync(importString, propertyType, options);
+
+        // When the original value was a non-string JSON type (array, object, number, etc.),
+        // convert string results back to JsonNode to preserve the correct JSON type
+        // and prevent double-encoding when the block value is re-serialized.
+        if (result is string stringResult && value.IsNonStringJsonValue())
+        {
+            return stringResult.ConvertToJsonNode() ?? result;
+        }
+
+        return result;
     }
 
     private async Task<object?> GetExportProperty(object? value, IPropertyType? propertyType, SyncSerializerOptions options)