  We study scaling behavior of the disorder parameter, defined as the
expectation value of a symmetry transformation applied to a finite region, at
the deconfined quantum critical point in (2+1)$d$ in the $J$-$Q_3$ model via
large-scale quantum Monte Carlo simulations. We show that the disorder
parameter for U(1) spin rotation symmetry exhibits perimeter scaling with a
logarithmic correction associated with sharp corners of the region, as
generally expected for a conformally-invariant critical point. However, for
large rotation angle the universal coefficient of the logarithmic corner
correction becomes negative, which is not allowed in any unitary conformal
field theory. We also extract the current central charge from the small
rotation angle scaling, whose value is much smaller than that of the free
theory.
