A Zariski dense exceptional set in Manin's conjecture: dimension 2

Recently, Lehmann, Sengupta, and Tanimoto proposed a conjectural construction of the exceptional set in Manin's Conjecture, which we call the geometric exceptional set. We construct a del Pezzo surface of degree $1$ whose geometric exceptional set is Zariski dense. In particular, this provides the first counterexample to the original version of Manin's Conjecture in dimension $2$ in characteristic $0$. Assuming the finiteness of Tate-Shafarevich groups of elliptic curves over $\mathbb{Q}$ with $j$-invariant $0$, we show that there are infinitely many such counterexamples.