PABretherton
I've analyzed your JSON.
The problem is related to the Base/Neck transform constraint that is enabled when Strap_01A, Strap_01B, or Strap_01C are enabled.
The Head bone is a child of the Neck bone, which is a child of Body_T1. Also note that Body_T2 is a child of Body_T1.
The Base/Neck transform constraint connects the source bone Body_T2 to the target bone Neck.
This means that when this transform constraint is enabled, the Base/Neck follows the transform of Body_T2, since in the JSON you sent us, all transform mixes are 100%.
The 2_Idle2 animation animates the scale of Body_T2, and consequently the scale of Neck and Head is changed too.
I bet that in the 4.3 editor you didn't connect the scale of the source to the scale of the target, but the JSON export from the 4.3 editor into a 4.2 JSON creates the transform constraint enabling all connections.
You can fix the issue by setting the mix of the scale X and Y to 0 in the 4.2 editor, then exporting the JSON again.