@vx9, thank you for the simplified asset, this was much easier to debug. There are two basic changes I made to your PG to get this working which you can see on the updated playground. The first is that I changed the method of tagging an animation group as additive from the AnimationGroup.makeAnimationAdditive method to the simpler accessor AnimationGroup.isAdditive = true. Setting this accessor to true will set all targeted animations within the group to targetedAnimation.isAddive = true automatically. This did make the animations add correctly, but that opened up the second fix for the asset.
Since both the idle animation and the turn head animation had sampled keys from the Blender export, there is data for all keyable channels that become eligible for adding. To describe this in the most simple terms, each bone in both of your animation groups have their scale set to (1.0, 1.0, 1.0). Since the animations were sampled for each frame, they all have a key on every frame setting the scale to (1.0, 1.0, 1.0). This means when you add the turn head animation group to your idle animation group, the scale for every bone will be (2.0, 2.0, 2.0). The same applies for every parameter - position, rotation, and scale - for any animations clips that are added.
This means that what you need do to is to delete any keys that should not add to the motion. To do this, delete any curve data on any bone in the turn head animation and only key the parameters that need to change on only the needed bones. In this case, I deleted all data from everything other than the head bone and on the head bone, I deleted translation and scale data.
When you export, you need to not sample your animations, which is what is baking keys to every frame. You do this by unchecking Always Sample Animations under the Animation group of the export parameters for glTF.
I will continue looking into AnimationGroup.makeAnimationAdditive to see if there is something that is particular to Blender exports that we need to account for as the meshes from the docs samples did not come from Blender. This should hopefully unblock you, but please feel free to ping with more questions if you have them.