Hi
Two things, have you checked the supply pressure to the tandem pump ? from memory about 6 to 8 psi.
Yes you can get leak off from the injectors.
The injectors sit inside a precise machined tube in the head. Sometimes the faces of this machining become very burred / scored and allow fuel by. If the injectors are slightly loose this happens.
Pop out the injectors and check.
Could be a new head and injectors if so.