Hi @Liu, Sandy,
What's the version of Word did you use?
I tied to test you document with your steps but I can reproduce this problem. Then I found if I copy the fields 2 first, then delete Filed 1 title content and ctrl + v to paste the content in Title 1, I can get the same result.
If I delete the Title 1 and then copy the fields 2 and ctrl + v to Title 1, the result will correct.
Delete Title 1:
Copy Field 2:
Ctrl + V to Title:
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