Check your wiring to see if there are 2 wires going onto the + terminal on the coil. One wire will be the lower voltage ,about 9V, ignition supply from the ignition "on" position, the second will be a 12V supply coming from the starter circuit when cranking only. My guess is the coil is not getting the 12v when cranking and only starts when the key returns to the on position and the coil finally gets power from the 9V.
The Pinto 2L has the 12V coil wire coming from a terminal on the starter motor solenoid.