  Near-infrared (NIR) observations of normal Type Ia supernovae (SNe Ia)
obtained between 150 to 500 d past maximum light reveal the existence of an
extended plateau. Here, we present observations of the underluminous,
1991bg-like SN 2021qvv. Early, ground-based optical and NIR observations show
that SN 2021qvv is similar to SN 2006mr, making it one of the dimmest,
fastest-evolving 1991bg-like SNe to date. Late-time (170-250 d) Hubble Space
Telescope observations of SN 2021qvv reveal no sign of a plateau. An
extrapolation of these observations backwards to earlier-phase NIR observations
of SN 2006mr suggests the complete absence of a NIR plateau, at least out to
250 d. This absence may be due to a higher ionization state of the ejecta, as
predicted by certain sub-Chandrasekhar-mass detonation models, or to the lower
temperatures of the ejecta of 1991bg-like SNe, relative to normal SNe Ia, which
might preclude their becoming fluorescent and shifting ultraviolet light into
the NIR. This suggestion can be tested by acquiring NIR imaging of a sample of
1991bg-like SNe that covers the entire range from slowly-evolving to
fast-evolving events ($0.2 \lesssim s_\mathrm{BV} \lesssim 0.6$). A detection
of the NIR plateau in slower-evolving, hotter 1991bg-like SNe would provide
further evidence that these SNe exist along a continuum with normal SNe Ia.
Theoretical progenitor and explosion scenarios would then have to match the
observed properties of both SN Ia subtypes.
