When the change was made so that seeds would stack, any newly harvested seeds that came from brightly colored plants no longer included the word "bright" in the seed label. So, for example, if you harvest seeds from a bright orange pampas grass plant, you end up with seeds that are bright orange in color but are called "orange pampas grass seeds." These seeds will usually stack with each other just fine. They will not, however, stack with seeds that have the same label but came from plain old orange pampas grass plants even though they have the same name because the color of the seeds is actually different and they will produce different colors of plants.
If you have old nonstacking seeds from bright orange pampas grass plants, they will each still be labeled as a "bright orange pampas grass seed." If you want to make those seeds stack and put them with any piles of seeds from the same type of plant that you harvested after the stacking change went in, you have two choices: (1) plant each of the seeds and then dump them out of their pots and then stack them; or (2) put a bag of those old nonstacking seeds labeled as "bright orange pampas grass seeds" in the gardener's toolbox and hope that what you get back is a nice tidy stack of "orange pampas grass seeds."
At this point, after you have all your pampas grass seeds from bright orange pampas grass plants in a tidy stack, you can cause yourself further unending work and more lockdowns by using poppy seed dust on the pile of "orange pampas grass seeds" from bright orange pampas grass plants so that they have a label that correctly identifies their source, i.e., they will now be labelled correctly as "bright orange pampas grass seeds." Beware, though, that if you continue to grow bright orange pampas grass plants and collect seeds from them, the newly harvested seeds will be incorrectly labeled as "orange pampas grass seeds" and won't stack with your lovely pile of "bright orange pampas grass seeds."
Confused yet?
So am I. All I know at this point is that it's a waste of time to use poppy seed dust on stacks of brightly colored seeds that don't say they are brightly colored seeds. Now, if you're color blind, maybe you should use the poppy dust.... Just be prepared for extra work until someone fixes this mess.