Let \(\mathcal {R}(n)\) denote the number of representations of the even integer n as the sum of two squares, a cube and three biquadrates of primes and we write \(\mathcal {E}(N)\) for the number of positive integers n satisfying \(n \le N,\) \(n \not \equiv \ 2 \pmod 3\) such that the expected asymptotic formula for \(\mathcal {R}(n)\) fails to hold. In this paper, we prove \(\mathcal {E}(N)\ll N^{\frac{5}{12}+\varepsilon }\) for any \(\varepsilon > 0.\) The exponent \(\frac{5}{12}\) constitutes a refinement of \(\frac{61}{144}\) due to Liu.