There is a 99.99997% chance that you have set up a composition with non-square pixels. If it were me, I would only work in Square pixel compositions using the standard presents. There is no reason anymore to ever make a master comp non square.
The other extremely small possibility is that your lawyer or footage is set to rectangular pixels.
Thankyou for the reply. Comp setting are square-pixel. I'm using someone's stock footage and not sure how I find out if the footage/layer is set to rectangular pixels? I had to go to Transform -> fit to comp size because the stock footage was massively wide (before I started masking). Could that have made a difference?
Okay you have to educate yourself on pixel aspect ratios. You need to check the interpretation of the footage in the project panel. You should never ever disproportionately scale footage to fit in a composition window unless you intentionally want to distort the footage. That is why your mask is egg shaped. Make sure the footage is properly interpreted then, if you need to scale, scale uniformly, then add your mask.
After Effects will properly interpret all non-square camera original footage if the interpretation is correct. You know if the footage is properly interpreted you need to know where the footage came from and the specifications for that camera and format. You have a little homework to do.
Thank you so much for your help.
The issue is resolved and I can get the mask to be the correct shape if I make the composition the same aspect ratio as the footage but the footage is 4k and at a ridiculous 4096 pixels wide by 1728 high. If I always have to scale uniformly do I just accept that certain edges of my video (that I might want) are going to be cropped away?