Sorry for the delay–I see that I have the same issue with versions 1.2.2, 1.3.10, and 1.4.0.
My problem does look pretty much the same as the linked one. It looks buggy when I view it with my OS’s built-in image viewer or Pix, normal here and if I just view it with Firefox, and even more messed up on Discord. Whereas if I render to PNGs and rerender into a GIF with GIMP, it looks normal everywhere. So it seems like there is a problem with how Synfig renders GIFs.