Commercial papers commonly are sized (coated) with gelatin or starch, both of which have an attraction for ferricyanide compounds. Adding these mild organic acids could increase this attraction by making the naturally acidic gelatin or starch even more acidic and thus even more likely to bind with the iron ferricyanide complex, (this also applies to ferrocyanide compounds). That would explain why the resulting print image is as strong or stronger than before.

I concur with the idea that these acids would overcome the buffers added to the paper and allow the reaction forming the cyan complex to proceed more aggressively. Lowering the ph in this way would not help the light sensitive reaction, which is Fe3+ =hv=> Fe2+, but it might make the ferricyanide more reactive, again making the reaction more aggressive.

In any case if the reaction proceeds more aggressively for whatever reason then more of the blue cyan complex will form--some even in liquid lying unbonded with the paper. More forms and there is more for the image and more to wash away in the clearing rinse.