Going to take a SWAG at this one ( Scientific Wild A$$ Guess )
Lets say the bags are 100% cotton, not sure they are just doesn't look like a cotton weave on the black bag BUT. What if they have some oil or other hydrocarbon spots in / on the fabric like some tee shirts we see
. What if the trip through the dryer vaporized or off gassed the substance whatever it might be changing the characteristics of the ink. So instead of dye migration it is something like "ink evacuation" or ink reaction leaving behind the spots.
On similar note take for example the new car smell you get, well, in a new car. That is not put there by the car maker it is a result of the plastics and materials in the interior of the car off gassing. A process that continues for some time long after the car value drops soon as you drive it off the lot.
The comment that the situation is not on all and the visible patterns are random kind of suggests some kind of foreign hitch hiker on / in the material. If you have more bags try an alcohol wipe on the material and a run through the dryer before you print.
mooseman