Are you doubling the mesh anywhere in the head when you compile>? Are you exporting SMD or DMX files and is the face solid and doesn't have any non manifold edges or vertexes?

It is a DMX. There were non manifold edges or vertexes, but when I removed them, the model still had distorted flexes in HLMV. A part of the the head was duplicated, before any flexes were made, in order to have black textured faces within the head so that when the mouth is opened, black can be seen.

By doulbling, I mean, do you have the head objects multiple build statements like

$model "blah" "head.dmx" {
blah...
blah...
}

and/or a

$body "head" "head.dmx"

and/or a

$bodygroup "head" {
studio head.dmx
studio blah...
studio blah...
blank
}

This is my QC. It is a very simple QC, body.dmx contains the whole model

$CD "D:\Desktop 12-20-19\Folders\Blender\Blender Exports\SFM Exports\Huck Rig Test"

$modelname "boomerang_from_cartoon_network\huckleberry_hound\rig_test\huck_rig_test.mdl"

$body "Body" "huck.dmx"

$mostlyopaque
$ambientboost

$cdmaterials "models\boomerang_from_cartoon_network\huckleberry_hound"

$sequence "idle" "huck.dmx" {
fps 1
}

There's only one other suggestion I can make without actually looking at the model myself and that is, when you exported the mesh, was the basis shape in Blender selected and were all the shapes zeroed?

Yeah. The Basis Shape Key is selected, and all of the flexes are zeroed. I still don't understand what you were getting at with the doubling, like how would that cause an issue?

If you have 2 or more of those statements in the QC (you don't) loading the same DMX then each of the statements creates an instance of the bodygroup. One for each statement it encounters. The shapekeys/flexes become confused (and usually don't work at all), because the multiples all have flexes with the same names. It all depends on which bodygroup the game engine thinks is the one that should have shapes/flexes.

Anyway, that is not the case in your case. You have a clean QC.

I would decompile the broken model and bring it back into Blender and have a look at each of the shapes to see if I could discover what is deforming by the key and what isn't.

This is strange. When the model was decompiled, multiple SMDs were created as well as VTAs. The model also appears to be divided up within all of the SMDs.

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2427089176

The model appeared to have too many vertices. What is the max vertices for a model again?

For body groups (BG) is around 9000 to 11000 verts per BG unless $maxverts xxxxx (where xxxxx is up to 65530) is used in the QC. But I haven't hit the max in a model yet adding BGs.

As for the multimesh SMDs, check your model for doubles (blender m (for merge) -by distance) watch out for sealing the mouth and eyes)