  Recent LHCb data shows that the direct CP asymmetries of the decay modes
$D^0\rightarrow \pi^+\pi^-$ and $D^0\rightarrow K^+K^-$ have the same sign,
violating an improved $U$-spin limit sum rule in an unexpected way at
$2.1\sigma$. From the new data, we determine for the first time the imaginary
part of the CKM-subleading, $U$-spin breaking $\Delta U=1$ correction to the
$U$-spin limit $\Delta U=0$ amplitude. The imaginary part of the $\Delta U=0$
amplitude is determined by $\Delta a_{CP}^{\mathrm{dir}}$. The corresponding
strong phases are yet unknown and could be extracted in the future from
time-dependent measurements. Assuming $\mathcal{O}(1)$ strong phases due to
non-perturbative rescattering, we find the ratio of $U$-spin breaking to
$U$-spin limit contributions to the CKM-subleading amplitudes to be
$(173^{+85}_{-74})\%$. This highly exceeds the Standard Model (SM) expectation
of $\sim 30\%$ $U$-spin breaking, with a significance of $1.95\sigma$. If this
puzzle is confirmed with more data in the future, in the SM it would imply the
breakdown of the $U$-spin expansion in CKM-subleading amplitudes of charm
decays. The other solution are new physics models that generate an additional
$\Delta U=1$ operator, leaving the $U$-spin power expansion intact. Examples
for the latter option are an extended scalar sector or flavorful $Z'$ models.
