Since 10.3.0 (including 10.4.0) we have experienced problems merging Word-documents where sometimes style is partially lost. I believe we have narrowed it down to when we are merging texts that include a space, like say a person's full name.
What happens is that if we for example have a merge field named FullName and that field is Arial 12 in the template and then we merge with "Firstname Surname", in the final PDF, "Firstname" will have the correct style whereas "Surname" will have something else.
We haven't yet been able to deduce the logic behind the style that the second word gets, but it might be caused by the style/font that surrounds the merge field in some way. What's certain though is that this only happens when merging text that have spaces.
We recently upgraded to 10.4.0 from 10.0.1 due to other problems we were having with that release, but today we had to downgrade to 10.2.0 after find this problem and also finding that we had the some problem with 10.3.0. With 10.2.0, we get the expected output.
I don't have a template to attach at this point, but you'd like to, I can try to create a "test case".