Observer DOES detect piston heads activate/deactivate.
As it is stated in your gamepedia site Here
"A piston (not the head) extending or contracting; Detected: PE=No|PC=Yes"
But it does detect the head.
As it is stated in your gamepedia site Here
"A piston (not the head) extending or contracting; Detected: PE=No|PC=Yes"
But it does detect the head.