I have a feeling that the Jerky Dryer was pulling too much air through the smoker, and may have even ignited your sawdust which made the A-MAZE-N smoker burn a little too hot, thus the color difference on your bottom shelf fish.
What positioning did you have your racks? Do you have the add-on Side-Rail assembly with the extra shelves (useful for low height items like Jerky/fish)? I love the this new side rail assembly because it lets me get my items closer to the top of the smoker where the heat levels are more stable.
If the bottom rack was too close to the bottom of the smoker, that would be the cause in my opinion.
When cold smoking, I always put my racks as high in the smoker as I can and I usually place a couple of 38" to 1/2" sticks of plywood under the Jerky Dryer to reduce the amount of air flow that the Jerky Dryer pulls through the smoker.
Another example, of why I think this may be causing the problem is that our standard recommendation for smoking Jerky is to smoke without the dryer for the first couple of hours (in other words during the main smoking phase), because the Jerky Dryer pulls too much air and could start the wood on fire.
Hopefully this helps.