  The quantum coherence and gate fidelity of electron spin qubits in
semiconductors is often limited by noise arising from coupling to a bath of
nuclear spins. Isotopic enrichment of spin-zero nuclei such as $^{28}$Si has
led to spectacular improvements of the dephasing time $T_2^*$ which,
surprisingly, can extend two orders of magnitude beyond theoretical
expectations. Using a single-atom $^{31}$P qubit in enriched $^{28}$Si, we show
that the abnormally long $T_2^*$ is due to the controllable freezing of the
dynamics of the residual $^{29}$Si nuclei close to the donor. Our conclusions
are supported by a nearly parameter-free modeling of the $^{29}$Si nuclear spin
dynamics, which reveals the degree of back-action provided by the electron spin
as it interacts with the nuclear bath. This study clarifies the limits of
ergodic assumptions in analyzing many-body spin-problems under conditions of
strong, frequent measurement, and provides novel strategies for maximizing
coherence and gate fidelity of spin qubits in semiconductors.
