  We compare the decay of the heavy Higgs boson into two SM-like Higgs bosons,
$H\to hh$, calculated in a Feynman-diagrammatic approach at the one-loop level
based on the one hand on the full effective potential involving the top quark
and stops in the Minimal Supersymmetric Standard Model (MSSM) accompanied by
the matched Two-Higgs-Doublet Model (2HDM) as its low-energy limit and on the
other hand on the hMSSM approximation. We identify missing contributions due to
the top quark in the Higgs self-couplings of the hMSSM, that - when taken into
account - lead to a good agreement between the hMSSM and a full MSSM
calculation, at least in the limit of the Higgsino mass parameter $\mu$ being
small compared to the stop spectrum. We also thoroughly analyze
momentum-dependent and kinetic corrections intrinsic to the
Feynman-diagrammatic approach and the matching to the effective Lagrangian,
respectively, for both our calculation in the MSSM and the hMSSM and for the
latter suggest to include additional corrections from the top quark, which are
independent of the unknown supersymmetric spectrum.
