  Decays based on the $b\to s\gamma$ transition are expected to yield
left-handed photons in the Standard Model, but could be particularly sensitive
to New Physics contributions modifying the short-distance Wilson coefficients
$\mathcal{C}_7$ and $\mathcal{C}_{7'}$ defined in the low-energy Effective
Field Theory. These coefficients can be determined by combining observables of
several modes, among which $B_s\to\phi ee$ at low $q^2$, in a kinematic range
where the photon-pole is dominant. We investigate the impact of $B_s-\bar{B}_s$
mixing on the angular observables available for this mode, which induces a
time-dependent modulation governed by interference terms between mixing and
decay that are sensitive to the moduli and phases of $\mathcal{C}_7$ and
$\mathcal{C}_{7'}$. These interference terms can be extracted through a
time-dependent analysis but they also affect time-integrated observables. In
particular, we show that the asymmetries $A_T^{(2)}$ and $A_T^{(\text{Im});CP}$
receive terms of potentially similar size from mixing-independent and
mixing-induced terms, and we discuss how the constraints coming from the
angular analysis of $B_s\to\phi ee$ at low $q^2$ should be interpreted in the
presence of mixing.
