  We present a JWST MIRI/MRS spectrum (5-27 $\mathrm{\mu}$m) of the Type Ia
supernova (SN Ia), SN 2021aefx at $+415$ days past $B$-band maximum. The
spectrum, which was obtained during the iron-dominated nebular phase, has been
analyzed in combination with previous JWST observations of SN 2021aefx, to
provide the first JWST time series analysis of an SN Ia. We find the temporal
evolution of the [Co III] 11.888 $\mathrm{\mu}$m feature directly traces the
decay of $^{56}$Co. The spectra, line profiles, and their evolution are
analyzed with off-center delayed-detonation models. Best fits were obtained
with White Dwarf (WD) central densities of $\rho_c=0.9-1.1\times 10^9$g
cm$^{-3}$, a WD mass of M$_{\mathrm{WD}}$=1.33-1.35M$_\odot$, a WD magnetic
field of $\approx10^6$G, and an off-center deflagration-to-detonation
transition at $\approx$ 0.5 $M_\odot$ seen opposite to the line of sight of the
observer (-30). The inner electron capture core is dominated by energy
deposition from $\gamma$-rays whereas a broader region is dominated by positron
deposition, placing SN 2021aefx at +415 d in the transitional phase of the
evolution to the positron-dominated regime. The formerly `flat-tilted' profile
at 9 $\mathrm{\mu}$m now has significant contribution from [Ni IV], [Fe II],
and [Fe III] and less from [Ar III], which alters the shape of the feature as
positrons excite mostly the low-velocity Ar. Overall, the strength of the
stable Ni features in the spectrum is dominated by positron transport rather
than the Ni mass. Based on multi-dimensional models, our analysis is consistent
with a single-spot, close-to-central ignition with an indication for a
pre-existing turbulent velocity field, and excludes a multiple-spot, off-center
ignition.
