Filling-in word form from access vba in readonly mode does not work in O365

I have an Access database with VBA for filling-in a word document in readonly mode. It does work properly in Office 2016 but does not work in O365. In Office 365 the fields remain empty whereas in Office 2016 the data is populated in the fields correctly.

The Word document has to be opened in readonly mode so that the original document cannot be changed.

Does anyone knows this issue or did I miss something?

Here es an example, how I open the word document:

Sub FillIn()
Dim WordApp As Word.Application
Dim Doc As Word.Document

Set WordApp = New Word.Application
WordApp.Visible = True

Set Doc = WordApp.Documents.Open(FileName:="C:\myWordTemplate.docx", ReadOnly:=True)
With Doc
    .FormFields("Firstname").Result = Me.Firstname
    .FormFields("Lastname").Result = Me.Lastname
End With

End Sub

Thanks for any help