Make sure the UnsignedCoinJoin goes through TransactionBuilder optimizations.
MODIFY
WalletWasabi/Models/ChaumianCoinJoin/CcjRound.cs
WalletWasabi/Models/ChaumianCoinJoin/CcjRound.cs
@@ -308,6 +308,7 @@ namespace WalletWasabi.Models.ChaumianCoinJoin
var builder = Network.CreateTransactionBuilder();
UnsignedCoinJoin = builder
.ContinueToBuild(transaction)
+ .AddCoins(spentCoins) // It makes sure the UnsignedCoinJoin goes through TransactionBuilder optimizations.
.BuildTransaction(false);
// 8. Try optimize fees.

Make sure the UnsignedCoinJoin goes through TransactionBuilder optimizations.