Saw this question come up on a previous troubleshoot and nothing was really figured out so I'll just ask again.
I made a mask in AE with the pen tool. For whatever reason, the area masked is outside the boundaries of what is designated by the path. I accidentally moved the physical position of the layer earlier and then fixed it, and though "Maybe it's just AE being weird about that, so I'll just remake the mask from scratch once everything's been re-aligned and see what happens."
Same issue. Played with the sliders a little bit too to see if the masked area with just re-render and fix itself. No luck.
Lower layer is same image in same position with same settings.
Running CC 2015. Windows 10. I7-5820 Processor. 32 GB RAM. GTX 970 graphics card. Ate cereal for breakfast this morning.
What exact version number of AE CC 2015 are you running? Is it 18.104.22.168?
What effects are applied to your layer?
I am looking at your screenshot on my phone and I can't see anything wrong. If you're not seeing a cash problem then the mask looks exactly lined up with the fuselage and wings of the airplane. I don't know what the mask is supposed to be doing and there is no easily visible transparency problem with the mask because we can't see transparency.
When you have a problem with a layer the first thing to do is to select that item and then press the u key twice to reveal all modified properties. Then you start turning things off one at a time until you find the problem.