I think there maybe a bug in Opera Mini when an image is sized using rem units within a flexbox layout. I've put a sample up here:

https://lamplightdev.com/operamini.html

The expected result is that the Opera logo should be the same dimensions as the box (as is the case in other browsers - including regular Opera). However if you visit the page in Opera Mini, then the image is not resized.

Is there somewhere I can file a bug report?

Many thanks,
Chris.