When a mass is identified on a mammogram, by definition it must be seen on two views. Hence, when we see a finding only on one view it is considered an assymetry (so it should most likely not be a mass )

Depending on how dense your breast are these “assymetries” require follow up diagnostic mammogram and/or ultrasound to exclude an underlying mass particularly in a woman with dense breast pattern as an underlying mass could be hidden in the orthogonal view under the dense pattern making the finding visible only on one view which could easily be dismissed.

It could be an artifact, but don't be surprised if you get called back for additional/spot compression views and/or possible ultrasound.
All the best.