Thank you for your response

Of course, I am using Ctrl+Alt+Enter to insert the style separator
To clarify, I use this technique to insert the style separator
Firstly, separate the two sentences by Enter, then putting the pointer at the end of the first sentence, and then pressing Ctrl+Alt+Enter.

You can download the attached file to explore the cause and what I mean

Hiding Style Separator Mark.docx - BayFiles