gistfile1.cs

// Problematic short example
var q = db.X.All().Join(db.Y, XId: db.X.Id).WithY();

// Problematic real example
var q = db.List.All().Join(db.Question, ListId: db.List.Id).WithQuestion();


// Suggested short example
var q = db.X.All().Join(db.Y, XId: db.X.Id ).With(db.X.Y);

// Suggested real example
var q = db.List.All().Join(db.Question, ListId: db.List.Id ).With(db.List.Question); // correct ?

OK, try

dynamic question;
var q = db.List.All().Join(db.Question, out question).On(question.ListId == db.List.Id ).With(question);

Your original code should work, though; I will find out why it doesn't.