The property of cross-reference link missing Bold when combine and print pdf

Hi Community,

I have many .fm documents and I have combined them all to make a book.

I found some the cross-reference link missing bold in the pdf after printing the book, even though it was set up. (It only happens for books, not when printing separate files)

Eg: The format of the cross-reference style.

<Bold><~Reference><$paranum>, <$paratext>

How to fix this error to keep property of cross-reference link when print book?