What does it mean if a new focal asymmetry on screening mammogram "flattened into benign pattern" on diagnostic mammogram? I have very dense breasts.

No abnormality. It generally means that the original appearance on the screening mammogram was due to a coincidental overlapping of normal breast structures, causing the false appearance of a possible abnormality. On the diagnostic images, the overlapping structures spread out and look normal again, proving that the original "abnormality" was not real. A true mass would not flatten or spread out on the diag mammo.