When you attempt to extend the size of an EagerZeroedThick VMDK from the vSphere Client, the extended part of the disk becomes LazyZeroedThick.

VMware vSphere ESXi 5.1
VMware vSphere ESXi 5.0
VMware vSphere ESXi 7.0.0
VMware vSphere ESXi 6.7
VMware vSphere ESXi 6.5
VMware vSphere ESXi 6.0

Workaround:

If you need to extend the VMDK and want the VMDK to be EagerZeroedThick, run this command:

~ # vmkfstools -X 6G -d eagerzeroedthick /vmfs/volumes/cs-ee-symmlun-001A/cormac.vmdk
Grow: 100% done.
All data on /vmfs/volumes/cs-ee-symmlun-001A/cormac.vmdk will be overwritten with zeros from sector <8388608> onwards.
Zeroing: 100% done.

If you extend the disk by using UI, you have no control over all the options to extend the disk. As a result, the EagerZeroedThick VMDK automatically becomes LazyZeroedThick.

Note: To implement the above changes in ESXi 6.x refer the below documents:

ESXi 6.0: https://docs.vmware.com/en/VMware-vSphere/6.0/com.vmware.vsphere.storage.doc/GUID-97FED1EC-35A5-4EF2-80BA-7131F8455702.html
ESXi 6.5: https://docs.vmware.com/en/VMware-vSphere/6.5/com.vmware.vsphere.storage.doc/GUID-97FED1EC-35A5-4EF2-80BA-7131F8455702.html
ESXi 6.7: https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.storage.doc/GUID-97FED1EC-35A5-4EF2-80BA-7131F8455702.html

For more information, see Determining if a VMDK is zeroedthick or eagerzeroedthick (1011170)