  Exchange-only spin qubits hosted in $^{28}$Si-based triple quantum dots do
not suffer from decoherence caused by randomly fluctuating nuclear-spin
ensembles and can be relatively robust against electrical noise when operated
at a sweet spot. Remaining sources of decoherence are qubit relaxation, leakage
out of the qubit subspace, and dephasing due to residual effects of charge
noise, the latter two of which are the focus of this work. We investigate
spin-orbit-mediated leakage rates to the three-spin ground state accompanied by
virtual (i) tunneling, (ii) orbital excitation, and (iii) valley excitation of
an electron. We find different power-law dependencies on the applied magnetic
field $B$ for the three mechanisms as well as for the two leakage rates,
ranging from $\propto B^5$ to $\propto B^{11}$, and identify the sweet spot as
a point of minimal leakage. We also revisit the role of electrical noise at the
sweet spot, and show that it causes a decay of coherent qubit oscillations that
follows a power law $\propto 1/t$ (as opposed to the more common exponential
decay) and introduces a $\pi/2$ phase shift.
