Word automatically switches to Times New Roman whenever I type using a German keyboard layout
This is an obscure bug that affects Windows 10 when the language is set to "English (German)" with a German keyboard layout.
The solution is in Settings to set the language to pure "English (US)", then add the German keyboard layout.
The reports I have found were specifically for that combination of language and the German keyboard, but this bug might possibly happen with other languages. I can only theorize that it relates to some mishandling in Word of such a language/keyboard combination.