It doesn't look like the x/y coordinates are in longitude/latitude (WGS 84) in the URL request:
inSR=4326
&outSR=26912
&geometries={"geometryType":+"esriGeometryPoint",+"geometries":+[{"x":+-12456757.481260095,+"y":+4974725.271437088}]} %0D%0A
&transformation=108190
&transformForward=true
&vertical=false
&f=html
Project documentation.