Why am I getting UV distortion on a flat surface? I have marked a seam around the entire border of a surface and when unwrapping I'm seeing odd distortion and scaling issues.

I've recorded my mesh to show the issue.

As you can see, there is distortion in the middle of the face and a scaling error in the bottom right "leg".

I have averaged and packed the UV. Does anyone have any idea what could be causing this? Thank you.
To get a nearly undistorted UV, the modeling topology should be made by even quads, with every pole made only of 4 edges (5 and 3 where strictly necessary), which is not your case. Anyway, as the surface is flat, you can set the 3D view in a front ortographic perspective, select all the faces of the flat surface and press U "Project from view" and you will get an undistorted UV. Then you can select all the UV islands in the UV editor and average and pack everything.