Thank you Donald.
You are correct.
Both hydroquinone and catechol, when used as single component developers like the ones I tested, produce overall and proportional stain, but do not produce the shadow separation that they produce when combined with phenidone (with the resulting additive effects). An additional consequence is a reduction in effective film speed. Another consequence is a reduction in developer activity.

The result with Kodak Tech Pan is an EI of about 25 and a D Log E curve like the one POTA produces.