Strange thing happening when trying to mask an image

Hi, I'm new here and new to Premiere Pro and video editing in general. Please don't go all Stackoverflow on me.

I'm attempting to mask an image to hide it behind the subject's hand/shoulder, but when I create the mask, a strange thing happens where instead of hiding the masked part of the object, the masked portion remains in front but mirrored along the horizontal axis. What on earth is causing this?