Normalized invariant yield of $\mathrm{Z} \rightarrow \mu^+\mu^-$ (left panel) and corresponding \Raa (right panel) as a function of rapidity, measured in 0--90\% Pb--Pb collisions at \fivenn. The vertical bars represent statistical uncertainties only. The horizontal extension corresponds to the rapidity bin width. The uncorrelated systematic uncertainties are reported as filled boxes. The \Raa correlated systematic uncertainty is displayed as a box on the unity line in the right panel. The CT14 (at NLO) pQCD proton--proton cross section is used as reference to compute \Raa. The results are compared with free-nucleon PDF (CT14) and with nuclear PDF (CT14+EPPS16 ) calculations. The free-nucleon PDF calculations are larger than unity as a consequence of the isospin effect, which is properly taken into account by all the calculations.