There are two aspects to the problem: - Dependency graph update for compositor preview was missing updates flush. Apparently, update for new frame style of update will take care of flushing, but not the update tagged style of update. This goes to a legacy dependency graph and is to be changed at some point, but not so close to the release. - Movie clips were missing from the compositor dependency graph. This fixes part of T66519. Reviewers: brecht Reviewed By: brecht Differential Revision: https://developer.blender.org/D5256