delzrm
commented
4 years ago 
^^ further investigation reveals this:
InterpolateVertexAttributes seems to be getting called with indexes being duplicated (degenerate triangles?)
putting this at the start of InterpolateVertexAttributes() fixes the issue. if (i0 == i1 || i0 == i2 || i1 == i2) { Debug.Log("degenerate!"); Debug.Log("i0: " + i0.ToString() + " i1: " + i1.ToString() + " i2: " + i2.ToString()); return; }
it would appear that my mesh had a couple of degenerate triangles in! whoops!!
https://drive.google.com/file/d/1PgrIRS4AE_ekia7p29VeiRp77NEGZgpR/view?usp=sharing
Describe the bug Decimating meshes like this with submeshes and colour data seems make several of the triangles black in colour, the mesh appears to have been decimated perfectly fine, its just the colour data that is broken...
To Reproduce import this unity package (unity 2019.3.13f1)
https://drive.google.com/file/d/1PgrIRS4AE_ekia7p29VeiRp77NEGZgpR/view?usp=sharing
decimate the mesh (use lossless, or whatever settings you like) notice some triangles look like they are missing, infact, they aren't missing, their colour data has just been set to black.
Expected behavior
Colour data would be retained!
Screenshots
Image1 (before decimate) Image2 (after decimate) - notice a few black polygons...
Environment (please complete the following information): Windows10 - unity 2019.3.13f1
Additional context This mesh is made out of many submeshes, it only contains Verts,Normals and Colours. DecimateBug.zip