Packed UV triangles don't correspond to the model triangles

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I've been trying to use blender-xatlas with Blender 3.5.0 (commit 9719fd696488) and have had this problem where the pack looks right but the large triangles in the atlas match the small triangles on the sphere and vice-versa:

I presume that there's some sort of problem going on where triangle IDs aren't tracked before and after the pack so that things get all confused.