Quasireal photons exchanged in relativistic heavy ion interactions are powerful probes of the gluonic structure of nuclei. The coherent 𝐽/𝜓 photoproduction cross section in ultraperipheral lead-lead collisions is measured as a function of photon-nucleus center-of-mass energies per nucleon (𝑊Pb𝛾⁢N) over a wide range of 40<𝑊Pb𝛾⁢N<400  GeV. Results are obtained using data at the nucleon-nucleon center-of-mass energy of 5.02 TeV collected by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 1.52  nb−1. The cross section is observed to rise rapidly at low 𝑊Pb𝛾⁢N, and plateau above 𝑊Pb𝛾⁢N≈40  GeV, up to 400  GeV, entering a new regime of small Bjorken-𝑥 (≈6×10−5) gluons being probed in a heavy nucleus. The observed energy dependence is not predicted by current quantum chromodynamic models.