This came out of the blue (no new graphics driver, no new font) on Ubuntu 16.04 (up to date) and only seems to affect certain applications and numbers AFAIK. For example in nautilus, the file manager, this is what "test 123456789" looks like:
It doesn't, for example, affect Firefox. (I assume it's only GTK applications.) It seems that at certain HiDPI scaling factors the numbers are rendered correctly. (changed via Ubuntu's "Displays" app)