Is Headhunter_V1.mdx the HD or SD asset?
Edit: I dug into this because I was intrigued and I found an explanation.
First, its important to note that because the headhunter model is super hunchbacked, the actual location of the chest attachment point is very similar to the head and overhead attachment locations... so you just have to deal with the way that looks.
Second, the problem isn't with the headhunter model directly; it's when using the berserker sub-animations of the headhunter model that the chest attachment point fails. When a headhunter upgrades into a berserker, it gets the "alternate" animation tag applied to it to change how the unit looks without actually doing a chaos morph (and the developers didn't know about reverse bear form so they didn't do that either) into another unit-type. You can see this by inspecting the model in the editor's viewer and tabbing through its various animations. For whatever reason, the headhunter model's chest attachment point has been removed from its 'alternate' animations. Therefore, trying to attach to chest will not show the effect until the 'alternate' tag is removed from the unit (which you can do with a trigger).
How do you fix this? Edit the model. Maybe the attachment point is there but has just been renamed when 'alternate' is applied? Not entirely sure because I'm not a model guy, but as I recall it should be possible to convert the model to .mdl and then view the animation name data by opening that file with a text (hex?) editor.