Polygon ear clipping fails for segments positioned on a line (?)

Hi, I'm trying to create an arrow from the polygon, and triangulation prints errors (in edit and when spawning an object) "Invalid polygon triangulation - no convex edges found. Your polygon is likely self-intersecting". Luckily for me, it still renders fine with this error (although you can see glitches on the video in some cases - but only when it is actually self-intersecting, so that's okay), and also in convex-only triangulation mode.

The video would be the best description: https://dl.dropboxusercontent.com/s/k56gycj3o7ym45z/2020-10-05_03-11-39.mp4

I tried to show that I don't have any self-intersections there, and that it only prints errors when those two segments are on the same line (but not intersecting). I haven't looked deeper or tried other polygon parameters to reproduce it, though.

Am I doing something wrong or does some math have to be fixed?

Notes:

- Using WebGL build, but that seems irrelevant

- Also have been reproduced on 2019.4.4.