Open 'Text > Text and Font?', select the font you want to
use as new
default, and click on the 'Set as default' button in the lower left
corner of the dialog.
The solution is correct, but I think there is also a bug around,
although I couldn't track the origin yet.
I've experienced that unexpected default font change a lot of times
and I think there is some operation that's mistakenly setting it.
Maybe it's copying and pasting styles or something. I'm not sure since
I couldn't find a way to reproduce it consistently.
My 2 cents.