This is expected behavior. You cannot directly clone or migrate an Eager Zeroed Thick format disk to Thick (Lazy Zeroed Thick).
As a workaround, you can clone or migrate the Eager Zeroed Thick disk to Thin, then once again from Thin to Thick. This can be performed live, via Storage vMotion.
Note: If a template or source machine disk was previously copied using more a traditional means, such as via Copy-DatastoreItem in PowerCLI, the Datastore Browser, or simply via copy commands in a shell session, this may be a cause as to why source files are Eager Zeroed Thick. Converting the source template or Virtual Machine to the desired format first can save deployment efforts, time, and storage.