From what I understand, this change was triggered by this discussion over at crowdin:
https://crowdin.com/project/factorio/discussions/195

I believe the actual font "Titillium" that Factorio shipped with previously had no support for cyrillic whatsoever, so the "old" font that you show in your ...

Hello. I would be glad if they returned the old style for the Russian (Cyrillic) text. With the new update (38), the font has become unreadable. Eyes get tired very quickly. Which is not good news, especially on a laptop. Yesterday, after installing the update, after 3-5 minutes my eyes got tired ...