  Constraints on inflationary $B$-modes using Cosmic Microwave Background
polarization data commonly rely on either template cleaning or cross-spectra
between maps at different frequencies to disentangle galactic foregrounds from
the cosmological signal. Assumptions about how the foregrounds scale with
frequency are therefore crucial to interpreting the data. Recent results from
the Planck satellite collaboration claim significant evidence for a
decorrelation in the polarization signal of the spatial pattern of galactic
dust between 353 GHz and 217 GHz. Such a decorrelation would suppress power in
the cross spectrum between high frequency maps, where the dust is strong, and
lower frequency maps, where the sensitivity to cosmological $B$-modes is
strongest. Alternatively, it would leave residuals in lower frequency maps
cleaned with a template derived from the higher frequency maps. If not
accounted for, both situations would result in an underestimate of the dust
contribution and thus an upward bias on measurements of the tensor-to-scalar
ratio, $r$. In this paper we revisit this measurement and find that the
no-decorrelation hypothesis cannot be excluded with the Planck data. There are
three main reasons for this: i) there is significant noise bias in cross
spectra between Planck data splits that needs to be accounted for; ii) there is
strong evidence for unknown instrumental systematics whose amplitude we
estimate using alternative Planck data splits; iii) there are significant
correlations between measurements in different sky patches that need to be
taken into account when assessing the statistical significance. Between
$\ell=55-90$ and over $72\%$ of the sky, the dust $BB$ correlation between 217
GHz and 353 GHz is $1.001^{+.004/.021}_{-.004/.000}$ ($68\%~stat./syst.$) and
shows no significant trend with sky fraction.
