Running Web Agent 12.52SP1CR10 build 2813, the samesite ACO parameter is set, but the Web Agent doesn't apply it.
Web Agent 12.52SP1CR10 build 2813 and lower version
Upgrade the Web Agent to 12.52SP1CR11, to get the functionality in the Web Agent. Pay attention to the support matrix and documentation on how to upgrade (1)(2)(3).