That may be because the iodine "neutralizer" tabs are vitamin C, which is contained in many drink mixes (ascorbic acid). Add it too soon and it will bind up the iodine before it has done its work.

I'd wait for treatment time to pass before adding to any chemical treatment, not just iodine. Let the stuff do its job with the fewest possible constituents in the water.

Cheers,
_________________________
--Rick