  In van der Waals heterostructures (HS) consisting of stacked MoSe$_2$ and
WSe$_2$ monolayers, optically bright interlayer excitons (ILE) can be observed
when the constituent layers are crystallographically aligned. The symmetry of
the monolayers allows for two different types of alignment, in which the
momentum-direct interlayer transitions are either valley-conserving (R-type
alignment) or changing the valley index (H-type anti-alignment). Here, we study
the valley polarization dynamics of ILE in magnetic fields up to 30~Tesla by
time-resolved photoluminescence (PL). For all ILE types, we find a finite
initial PL circular degree of polarization ($DoP$) after unpolarized excitation
in applied magnetic fields. For ILE in H-type HS, we observe a systematic
increase of the PL $DoP$ with time in applied magnetic fields, which saturates
at values close to unity for the largest fields. By contrast, for ILE in R-type
HS, the PL $DoP$ shows a decrease and a zero crossing before saturating with
opposite polarization. This unintuitive behavior can be explained by a model
considering the different ILE states in H- and R-type HS and their selection
rules coupling PL helicity and valley polarization.
