So... I investigated this issue today, and it is not a viewer bug, but an issue with badly designed meshes that happen to show fine once "optimized" (decimated) by LL's mesh vertex cache (buggy !) algorithm...
Let me elaborate:
LL's algorithm for optimizing vertex cache happens to work under Windows, but fails under Linux (and most probably macOS as well). Like Nicky Dasmijn explains in Firestorm's sources:
As a result, the optimization code is disabled for Linux in Firestorm... And you get this:
I.e. exactly the same result as with the current Cool VL Viewer version, because I disabled the vertex simplification pass with the "meshoptimizer" library (due to bad results with other meshes) that replaces LL's buggy code.
Fact is that this particular hair item is simply badly designed (since it should display properly without any optimization pass, and it does not), and its creator likely only tested it with LL's viewer under Windows (where the hair shows fine).
In older releases of the Cool VL Viewer, where the "meshoptimizer" library was not used, I had a (partial) fix implemented for the bogus sorting in LL's algorithm, and it appears that it did a better job than "meshoptimizer" (even if still not perfect under Linux). I will therefore reimplement LL's old algorithm with that fix, and you will get this with v1.28.2.20:
Linux:
Windows:
As for the "Leo" hair, it displays fine under both Linux and Windows with that partially fixed algorithm...
This said, when you encounter such problematic meshes, you should report them to their creator; the latter should really test properly their meshes and make sure there is no degenerate vertices and other glitches that might end up showing up depending on the viewer they are rendered with, or even to the settings in use (LOD level, etc)...