  Modified gravity theories can accommodate exact solutions, for which the
metric has the same form as the one in general relativity, i.e., stealth
solutions. One problem with these stealth solutions is that perturbations
around them exhibit strong coupling when the solutions are realized in
degenerate higher-order scalar-tensor theories. We show that the strong
coupling problem can be circumvented in the framework of the so-called U-DHOST
theories, in which the degeneracy is partially broken in such a way that
higher-derivative terms are degenerate only in the unitary gauge. In this
sense, the scordatura effect is built-in in U-DHOST theories in general. There
is an apparent Ostrogradsky mode in U-DHOST theories, but it does not propagate
as it satisfies a three-dimensional elliptic differential equation on a
spacelike hypersurface. We also clarify how this nonpropagating mode, i.e., the
"shadowy" mode shows up at the nonlinear level.
