Add Image to Blender's VSE fails to display alpha channel transparency In Blender's VSE (Video Sequence Editor), on trying to overlay a background video or image sequence with multiple foreground images (each containing transparency, or alpha channel, information), transparency is intermittently lost. Some images are shown with a black background, others retain their transparency.

Possible reasons?

NOTE: This question has been posted so that I can provide an answer derived from my own tests. As the question relates specifically to VSE, it has nothing to do with answers addressing 'Import Image as Plane', those referencing use of the RGBA output setting (which no longer appears to exist), nor those relating to alpha over / alpha under settings - which under these conditions have no effect.
Blender won't handle an alpha channel in a codec. Alpha in a codec is largely a nonstandard practice.