Vertices Not Indexed Properly?

[maxattack]

Some vertices appear indexed improperly (a handful out of ~1k, so it's <1%?)

No triangulation going on. However, even so, the indexing code is hard to follow as I debug this issue. Is there any guidance you can give me for possible leads?

(Left: OpenFBX imported geometry, Right: same FBX imported into maya)

[maxattack]

No malformed-data warnings importing into maya or anything.

[nem0]

Without triangulation indices/vertices are basically raw data from fbx, so it's probably something else. The code for that was contributed by some other user, so I don't guarantee it's correct. If you want you can upload the model and I will take a look.
Note: raw indices have negative values marking end of polygon: https://github.com/nem0/OpenFBX/blob/master/src/ofbx.cpp#L2299

[maxattack]

Yeah, I'm accounting for those. BTW, (-idx - 1) is just bitwise complement, so you can use ~idx and save some instructions.

mecha.zip

[nem0]

Note: singed integer binary representation before c++20 is implementation defined

[nem0]

I tried to import the model in Lumix Engine and it seems ok (although it's with triangulation)