Whirlpool coinjoins consist of two transactions. The traceable change is created from the tx0 premix transaction. While tx0 is already a huge waste of block space on its own, the change output created here is especially wasteful because it cost the user more in mining fees to create it than it is worth.
But what software did you use that produced this? I have never created a dust output like that, but have used whirlpool from sparrow multiple times.
A coinjoin contains inputs from multiple users. In Whirlpool, each user pays the coordinator fee in the tx0 transaction, then coinjoins with other peers in the following transaction. Since all peers are paying their fees to the same coordinator, they would save block space by pooling their payments into the creation of one output for the entire group as opposed to each participant creating an individual output.
As I understand it, participants pay during their entry. They cannot pay all together, because each one enters in a different time. It can happen in theory, but it would need to change their entire protocol.
