phpdocx integrates a template compatible with all MS Word versions. This provokes that some MS Word versions show [Compatibility Mode] with the file name in the title bar. This is a default MS Word behavior to try to obtain the best compatibility when opening documents generated with versions older than the current one.
To remove that text when starting MS Word, just indicate a MS Word version. You can do this with the docxSettings method and the compat option, setting the MS Word version number:
Or using a base blank template generated with the version of MS Word you want to work with.