Appreciate you continuing to look at this.
Is your line .png being accessed from the .fla library, or as a loose file? (If you are using MovieClip::loadMovie on a loose asset (as I am), can you provide a .zip of your test.fla so I can narrow down the difference?)
I did another test in a new .fla just to publish the results in Flash player, and the loaded .png appears fine. Without getting into too much detail, I am leveraging the .swf using Scaleform; I believe the effect I am experiencing is being caused by outside forces. I'm going to investigate the problem further, but I do believe that Flash is behaving as expected.