  We present a reconstruction of the semileptonic decays $B^0 \to \pi^- e^+
\nu_e$ and $B^+ \to \pi^0 e^+ \nu_e$ in a sample corresponding to 189.3
fb$^{-1}$ of Belle II data, using events where the partner $B$-meson is
reconstructed from a large variety of hadronic channels via a tagging algorithm
known as the full-event-interpretation. We determine the partial branching
fractions in three bins of the squared momentum transfer to the leptonic system
using fits to the distribution of the square of the missing mass. The partial
branching fractions are summed to determine $\mathcal{B}(B^0 \to \pi^- e^+
\nu_e)$ = (1.43 $\pm$ 0.27(stat) $\pm$ 0.07(syst)) $\times 10^{-4}$ and
$\mathcal{B}(B^+ \to \pi^0 e^+ \nu_e)$ = (8.33 $\pm$ 1.67(stat) $\pm$
0.55(syst)) $\times 10^{-5}$. We extract a first Belle II measurement of the
magnitude of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{\mathrm{ub}}|$,
with $|V_{\mathrm{ub}}|$ = (3.88 $\pm$ 0.45) $\times 10^{-3}$.
