diff --git a/.changeset/tabbed-posts-category-fix-scoped.md b/.changeset/tabbed-posts-category-fix-scoped.md
new file mode 100644
index 000000000..1ed005ce6
--- /dev/null
+++ b/.changeset/tabbed-posts-category-fix-scoped.md
@@ -0,0 +1,20 @@
+---
+"@devgateway/dvz-ui-react": patch
+---
+
+Revert the PR #296 fix for TCDICORE-601, which comma-joined all array/object
+attributes in `PreviewComponent.tsx` before postMessage'ing them to the
+block-editor preview iframe. That change was a shared choke point used by
+every embeddable block, and it broke any block whose attribute is an array of
+objects (e.g. `filters`, map `legendBreaks`) by turning it into an
+unparseable `"[object Object],[object Object]"` string.
+
+`PreviewComponent.tsx` now always JSON-stringifies object/array attributes
+again, restoring pre-PR-296 behavior for every block except Tabbed Posts.
+
+The original TCDICORE-601 problem — the Tabbed Posts editor preview sending
+`categories` as `"[5,7]"` instead of the front-end's comma-separated `"5,7"`
+— is now fixed locally inside the Tabbed Posts embeddable component
+(`tabbedposts/index.jsx`), which detects a JSON-array-shaped `categories`
+string and normalizes it to the comma-separated format the REST API expects,
+without changing how any other block's attributes are serialized.
diff --git a/packages/dvz-ui/src/embeddable/tabbedposts/index.jsx b/packages/dvz-ui/src/embeddable/tabbedposts/index.jsx
index ca84c7f70..35a688385 100644
--- a/packages/dvz-ui/src/embeddable/tabbedposts/index.jsx
+++ b/packages/dvz-ui/src/embeddable/tabbedposts/index.jsx
@@ -269,10 +269,20 @@ const Wrapper = (props) => {
     previewMode = pageModuleProps.previewMode;
     editing = pageModuleProps.editing;
   }
-  const locale = props.intl.locale;
-  const normalizedCategories = Array.isArray(categories)
-    ? categories.join(',').toString()
-    : categories;
+  const locale = props.intl.locale; 
+  let normalizedCategories = categories;
+  if (Array.isArray(categories)) {
+    normalizedCategories = categories.join(',');
+  } else if (typeof categories === 'string') {
+    try {
+      const parsedCategories = JSON.parse(categories);
+      if (Array.isArray(parsedCategories)) {
+        normalizedCategories = parsedCategories.join(',');
+      }
+    } catch (e) {
+      // Not JSON — already a plain string (e.g. "5,7"), use as-is.
+    }
+  }
 
   const scrollable = useScrolls === 'true';
   const conditionalHeight = scrollable ? height : undefined;
diff --git a/packages/dvz-ui/src/layout/PreviewComponent.tsx b/packages/dvz-ui/src/layout/PreviewComponent.tsx
index 51be2cc5c..a0088b656 100644
--- a/packages/dvz-ui/src/layout/PreviewComponent.tsx
+++ b/packages/dvz-ui/src/layout/PreviewComponent.tsx
@@ -28,9 +28,7 @@ const PreviewComponentParameterParser = () => {
             const newParams: Record<string, string> = { ...paramPropsRef.current };
             Object.keys(data).forEach(k => {
                 newParams["data-" + k.replace(/([a-z])([A-Z])/g, '$1-$2').toLowerCase()] =
-                    typeof data[k] === 'object'
-                        ? (Array.isArray(data[k]) ? data[k].join(',') : JSON.stringify(data[k]))
-                        : data[k];
+                    typeof data[k] === 'object' ? JSON.stringify(data[k]) : data[k];
             });
             setParamProps(newParams);
         }