Since Last Update I've Lost the Ability to View MP4 / MKV Files In Viewer Pane

12.33 had no changes to video playback. It's more likely something else on the system changed, probably to do with video codecs.

For Opus 12: HOW TO: Enable/fix playback of various media formats in Opus

For Opus 13: Directory Opus 13 - Highlights - Page 4 [Directory Opus Manual] has a list of recommended codecs for the new video player in 13.