I am using the Microsoft Office 2016 Beta Preview (Microsoft Word 15.13.1) on OS X Yosemite (10.10.5), but as far as I can recall, this issue has been present for a long time.
I have my Normal.dotm set up so that the
Normal paragraph style uses Minion, which fully supports the Greek Extended range. This works fine in general: when opening a new document, the default font is Minion.
When I type something in Modern Greek (i.e., Greek letters with only acute accents), the font remains Minion with no problems.
However, when I type more or less anything from the Greek Extended range (vowels with circumflex accents like ᾶῶῆ and/or rough or smooth breathing marks like ἀἁἄἅἆἇ, etc.), Word stubbornly—and extremely annoyingly—insists on immediately switching the font to Calibri, despite the fact that Minion contains the necessary glyphs.1 I then have to manually reapply the relevant paragraph style for every paragraph that happens to have such a glyph in it for it to display properly in Minion.
Why does this happen? Precisely which range of characters provokes this infuriating behaviour? And most importantly, is there any way to prevent it from happening?
1 It doesn’t happen with all these characters. Grave accents usually go through just fine, and at least some vowels with only a breathing mark also go through fine; but circumflexes nearly always breaks it, and an iota subscript is sure to do so.