The quick response you gave was very impressive, thank you very much @wupkube .
The Gutentor plugin is actually belongs to the same company. You may check it out here https://www.acmeit.org/
So it is very possible that the theme used some special block or filters from their own. Unfortunately they didn’t reply timely after I created a tech support ticket – either they are too busy in developing their products or they are short of manpower to do the customer service.
Anyways, I probably need to switch to another theme since I need the function badly. So, don’t bother to work on that theme if it doesn’t mean too much to you.
Thank you very much,
Jun