A family of Microsoft word processing software products for creating web, email, and print documents.
I've had a similar issue in my Word for Mac (Version 15.23), and none of the above fixes have worked for me.
What happens (in every single document) is that after Word autoformats something (-- into an m-dash, or changing a straight quotation mark into a curved one), it changes the font into Helvetica (from, say, Times New Roman).
This is particularly vexing after I copy-and-paste. If the copied text has something that Word autoformats, it then changes some of the following copied text to Helvetica-- AND THEN that font cannot be changed (e.g., if I select the Helvetica text, and try to use the font drop-down menu to change it to Times New Roman, it will stay in Helvetica. The only way I've been able to change the font at this point is to use the Format Painter tool-- but that's an irritating work-around).
I do not have any add-ins, and my default font is something else entirely (Calibri), so I'm not sure where to go from here.