A VM moving from one ESXi host to another (Whether using VMware Clone, vMotion, or a BC/DR solution including Zerto Virtual Replication) can cause the Microsoft Windows license mechanism to require re-activation.  This behavior is unfortunately expected as the Microsoft Licensing Briefs documentation link provided below, discusses some of these scenarios depending on the licensing model you may be using.

Microsoft Licensing Briefs: Virtualization

After the re-location of a VM from one physical host to another, the Microsoft Licensing mechanism sees that the the server hardware has changed and requires the Windows license to be re-activated. Please note that is a general issue not specifically limited to Zerto because while Zerto is hardware agnostic, the Windows operating system is not hardware agnostic by design.

After a Live Failover, some Windows servers may require Microsoft Windows license re-activation. 

It is possible to create a script to automate the license activation upon recovery if needed. For more information on running scripts before or after recovery of a VM, please review the section "Running Scripts Before or After Recovering a VPG" within the "Zerto Administration Guide" provided below. 

Zerto Administration Guide

Note: If additional clarification on why specific VMs require license re-activation, it may also be advisable to contact Microsoft support.