diff --git a/src/main/frontend/worker/pipeline.cljs b/src/main/frontend/worker/pipeline.cljs
index 018e03dcd6..72b9bb4376 100644
--- a/src/main/frontend/worker/pipeline.cljs
+++ b/src/main/frontend/worker/pipeline.cljs
@@ -125,9 +125,12 @@
                     (not (ldb/inline-tag? (:block/raw-title entity) tag))
                     (not (:db/ident entity)))
                (let [eid (:db/id entity)]
-                 [[:db/add eid :db/ident (db-class/create-user-class-ident-from-name db-after (:block/title entity))]
-                  [:db/add eid :logseq.property.class/extends :logseq.class/Root]
-                  [:db/retract eid :block/tags :logseq.class/Page]])
+                 (if (:block/page entity)
+                   ;; Built-in #Tag should never turn a page child block into a class.
+                   [[:db/retract eid :block/tags :logseq.class/Tag]]
+                   [[:db/add eid :db/ident (db-class/create-user-class-ident-from-name db-after (:block/title entity))]
+                    [:db/add eid :logseq.property.class/extends :logseq.class/Root]
+                    [:db/retract eid :block/tags :logseq.class/Page]]))
 
              ;; remove #Page from tags/journals etc.
                (= (:db/id page-tag) (:v datom))
diff --git a/src/test/frontend/worker/pipeline_test.cljs b/src/test/frontend/worker/pipeline_test.cljs
index 18587c1e04..c64753063b 100644
--- a/src/test/frontend/worker/pipeline_test.cljs
+++ b/src/test/frontend/worker/pipeline_test.cljs
@@ -3,6 +3,7 @@
             [datascript.core :as d]
             [frontend.worker.pipeline :as worker-pipeline]
             [logseq.db :as ldb]
+            [logseq.db.common.order :as db-order]
             [logseq.db.test.helper :as db-test]))
 
 (deftest test-built-in-page-updates-that-should-be-reverted
@@ -143,3 +144,46 @@
                                    :block/title "page1-renamed"}])]
         (is (= "page1-renamed"
                (:block/title (d/entity (:db-after result) (:db/id page1)))))))))
+
+(deftest built-in-tag-must-not-convert-page-child-block-to-class-test
+  (let [conn (db-test/create-conn-with-blocks
+              {:pages-and-blocks [{:page {:block/title "page1"}}]})
+        page1 (ldb/get-page @conn "page1")
+        now (js/Date.now)
+        bad-block-uuid (random-uuid)
+        new-tag-uuid (random-uuid)]
+    (ldb/register-transact-pipeline-fn! worker-pipeline/transact-pipeline)
+
+    (testing "page-child block with built-in #Tag stays a block"
+      (ldb/transact! conn [{:block/uuid bad-block-uuid
+                            :block/title "charlie"
+                            :block/created-at now
+                            :block/updated-at now
+                            :block/page (:db/id page1)
+                            :block/parent (:db/id page1)
+                            :block/order (db-order/gen-key)
+                            :block/tags [:logseq.class/Tag]}])
+      (let [block (d/entity @conn [:block/uuid bad-block-uuid])]
+        (is (some? block))
+        (is (nil? (:db/ident block)))
+        (is (nil? (:logseq.property.class/extends block)))
+        (is (not (ldb/class? block)))
+        (is (= (:db/id page1) (:db/id (:block/parent block))))
+        (is (empty? (:block/tags block)))))
+
+    (testing "standalone candidate is still converted to a class page"
+      (ldb/transact! conn [{:block/uuid new-tag-uuid
+                            :block/name "standalone-tag"
+                            :block/title "standalone-tag"
+                            :block/created-at now
+                            :block/updated-at now
+                            :block/tags [:logseq.class/Tag]}])
+      (let [tag-page (d/entity @conn [:block/uuid new-tag-uuid])]
+        (is (ldb/class? tag-page))
+        (is (keyword? (:db/ident tag-page)))
+        (is (= "user.class" (namespace (:db/ident tag-page))))
+        (is (= [:logseq.class/Root]
+               (map :db/ident (:logseq.property.class/extends tag-page))))))
+
+    ;; return global fn back to previous behavior
+    (ldb/register-transact-pipeline-fn! identity)))