I noticed this whist working on antialiased ellipses in #13825, which do not have this issue.
Ellipses drawn with the color #9696fb3e
Both these non-aa ellipses should be a uniform pale purple colour (like the bulk of the filled ellipse).
The filled variant has overlapping pixels on the right, and the outline variant has many overlapping pixels (that coincidentally makes it look like it's anti-aliased).
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