I make hundreds of pots from newspaper, (its the best use for The Western morning news) IMO but i make them round, so air space between, i have never put anything in a plastic bag, or a propogator, never owned one, maybe thats why yours have a build up now.
I have found that its mainly thomson and morgan that like to put everything in plastic bags, i have grown same flowers from 2 companys, the other not mentioning bags.
I use them for anything deep rooted, such as onions, leeks, sweet peas, cosmos, anything where you can put one seed in one pot. I dont use seed compost, just a very good multi purpose, then they can stay in there without having to be re potted, so no root disturbance. I find newspaper rots down ok in the wet soil when planted out.