  We use the light curve and spectral synthesis code JEKYLL to calculate a set
of macroscopically mixed Type IIb supernova (SN) models, which are compared to
both previously published and new late-phase observations of SN 2020acat. The
models differ in the initial mass, the radial mixing and expansion of the
radioactive material, and the properties of the hydrogen envelope. The best
match to the photospheric and nebular spectra and lightcurves of SN 2020acat is
found for a model with an initial mass of 17 solar masses, strong radial mixing
and expansion of the radioactive material, and a 0.1 solar mass hydrogen
envelope with a low hydrogen mass-fraction of 0.27. The most interesting result
is that strong expansion of the clumps containing radioactive material seems to
be required to fit the observations of SN 2020acat both in the diffusion phase
and the nebular phase. These "Ni bubbles" are expected to expand due to heating
from radioactive decays, but the degree of expansion is poorly constrained.
Without strong expansion there is a tension between the diffusion phase and the
subsequent evolution, and models that fit the nebular phase produce a diffusion
peak that is too broad. The diffusion phase lightcurve is sensitive to the
expansion of the "Ni bubbles", as the resulting Swiss-cheese-like geometry
decreases the effective opacity and therefore the diffusion time. This effect
has not been taken into account in previous lightcurve modelling of
stripped-envelope SNe, which may lead to a systematic underestimate of their
ejecta masses. It should be emphasized, though, that JEKYLL is limited to a
geometry that is spherically symmetric on average, and large-scale asymmetries
may also play a role. The relatively high initial mass found for the progenitor
of SN 2020acat places it at the upper end of the mass distribution of Type IIb
SN progenitors, and a single star origin can not be excluded.
