Of all the dye jobs I have done on FRN and G10 I have yet to have any trouble with mine. Zero issues. I would maybe check your water to vinegar ratio. We have really hard water here so I always use bottled water and very little of it, and a splash of Dawn soap.
More vinegar than anything and I always bring the vinegar and splash of water to a boil before stirring in the Dawn and boil again, then I add the dye after I remove from the stove...
Now if your sure on all those things I would probably question the FRN your knives are made from. Just me here, but in no way would I want to spray a polypropylene coating or anything over my scales either, that just sounds, no it screams at me something off...