Differential cross section of coherent \KK photoproduction as a function $\it{M_\mathrm{KK}}$ in Pb--Pb UPCs at \mbox{\snn = 5.02 \TeV} in $|y_\mathrm{KK}|\ \ 0.8$. The vertical lines and boxes along the data points represent statistical and systematic uncertainties, respectively. The green solid line presents the best fit result of \mbox{$|B_{\rm{KK}}/A_{\phi}|=0.28$ \GeVmassSR} and $\varphi$ = 0.06 rad together with the 1$\sigma$ bounds of the fit in a green band. The blue dashed curve shows the best fit with $|B_{\pi\pi}/A_{\rho}|=0.54$ \GeVmassSR~ and $\varphi$ = 1.46 rad (the best-fit values for $\rho$ plus direct $\pi^+\pi^-$)~. The black dotted line represents the curves under the hypothesis of $|B_{\rm{KK}}/A_{\phi}|=0$, showing only the $\phi (1020)\rightarrow$ \KK contribution. The gray band indicates the impact of the systematic uncertainty from the $\phi (1020)$ meson cross section, showing a 25$\%$ variation.