There is a font difference in the Viewable Rendition in Vault and the corresponding Microsoft Word source file that is uploaded by a user. The font in the source document is Calibri (Body), however, in the Viewable Rendition, it appears to be Times New Roman.
The default font is expected to apply to any style that does not have a defined font. Therefore, with the incomplete style used in the document, the font is based on the machine default, as it is not explicitly defined in the file. However, that does not apply to Word built-in heading styles, as their fonts are explicitly defined.
The solution is to clearly define all fonts in the source document prior to uploading to Vault.
Vault Renditions Documentation: Viewable Renditions