For a solo/solo flawless to count it must be done completely fresh in a single session. No boss checkpoints, no going to orbit to go back in. You can go afk if you must, but do know there is an idle timer that will kick you to orbit if you don't move for a few minutes.

It says your solo run was a checkpoint. I'm a little tired so my explanation might be a bit weird but there's also a checkpoint at the start of a dungeon meaning if you started and backed out it would save it which has been known to then cause this since it doesn't go away till weekly reset I believe.