diff --git a/addons/project/models/project.py b/addons/project/models/project.py
index 6eb5bf50fdb..6c67bc69a2d 100644
--- a/addons/project/models/project.py
+++ b/addons/project/models/project.py
@@ -1451,8 +1451,9 @@ class Task(models.Model):
             (In other words, this compute is only used in project sharing views to see all assignees for each task)
         """
         if self.ids:
-            # fetch 'user_ids' in superuser mode (and override value in cache)
-            self._read(['user_ids'])
+            # fetch 'user_ids' in superuser mode (and override value in cache
+            # browse is useful to avoid miscache because of the newIds contained in self
+            self.browse(self.ids)._read(['user_ids'])
         for task in self.with_context(prefetch_fields=False):
             task.portal_user_names = ', '.join(task.user_ids.mapped('name'))
 
diff --git a/addons/project/tests/test_project_sharing.py b/addons/project/tests/test_project_sharing.py
index 85a08d6beee..c995cf1d801 100644
--- a/addons/project/tests/test_project_sharing.py
+++ b/addons/project/tests/test_project_sharing.py
@@ -141,6 +141,7 @@ class TestProjectSharing(TestProjectSharingCommon):
             3) Give the 'edit' access mode to a portal user in a project and try to create task with this user.
             3.1) Try to change the project of the new task with this user.
             3.2) Create a sub-task
+            3.3) Create a second sub-task
         """
         # 1) Give the 'read' access mode to a portal user in a project and try to create task with this user.
         with self.assertRaises(AccessError, msg="Should not accept the portal user create a task in the project when he has not the edit access right."):
@@ -190,6 +191,12 @@ class TestProjectSharing(TestProjectSharingCommon):
             .create({'name': 'Test'})
         self.assertFalse(task2.portal_user_names, 'the portal user should not be assigned when the portal user creates a task into the project shared.')
 
+        # 3.3) Create a second sub-task
+        with self.get_project_sharing_form_view(task, self.user_portal) as form:
+            with form.child_ids.new() as subtask_form:
+                subtask_form.name = 'Test Subtask'
+        self.assertEqual(len(task.child_ids), 2, 'Check 2 subtasks has correctly been created by the user portal.')
+
     def test_portal_user_cannot_see_all_assignees(self):
         """ Test when the portal sees a task he cannot see all the assignees.