Imported video files show as expected in the preview window, but imported PNG file sequence does not. I am able to see rendered previews but the unrendered images do not appear. This is new in version 5.4.0
Thank you for writing in about this. A note has been sent to the devs with screenshots of it being replicated for their review to see why this is happening. They are going to be working on a patch for this.
Thanks, Kyle. Because it seems here to only effect a still sequence, it may have slipped past the team on the way to release. Looking forward to the patch.