Seen in browser tests across all browsers:
https://integration.wikimedia.org/ci/job/browsertests-MultimediaViewer-en.wikipedia.beta.wmflabs.org-linux-chrome-sauce/517/testReport/junit/%28root%29/Download%20menu/Image_size_defaults_to_original/
https://integration.wikimedia.org/ci/job/browsertests-MultimediaViewer-en.wikipedia.beta.wmflabs.org-windows_7-internet_explorer-9-sauce/362/testReport/junit/%28root%29/Download%20menu/Image_size_defaults_to_original/
https://integration.wikimedia.org/ci/job/browsertests-MultimediaViewer-en.wikipedia.beta.wmflabs.org-linux-firefox-sauce/518/testReport/junit/%28root%29/Download%20menu/Image_size_defaults_to_original/
https://integration.wikimedia.org/ci/job/browsertests-MultimediaViewer-en.wikipedia.beta.wmflabs.org-os_x_10.9-safari-sauce/521/testReport/junit/%28root%29/Download%20menu/Image_size_defaults_to_original/
The download menu itself seems to work fine on beta, though. Which suggests that the issue might be in the tests themselves.