Hello, I've just updated to Calibre 6.12 and now all books' fonts and images looks low-res or 'broken' (don't know the proper term). Attached are some screenshots.
https://puu.sh/Jyp3O/5757d0b90c.png
https://puu.sh/Jyp3U/3ee6cb81c9.png
https://puu.sh/Jyp3Z/fd5c7da07e.png
https://puu.sh/Jyp45/f182154e86.png