For certain workloads the Guest OS issued unmap can fail due to resource contention on ESXi side.
UNMAPs/ WRITE SAME issued by Guest OS need to lock a given resource cluster on the VMFS file system and if the same resource cluster is locked by a different work flow such as space allocation, then unmap request fails with resource contention.
Issue is not seen always and will be seen along with other workload that can cause resource contention for the unmap request.
This issue is fixed on VMware ESXi 7.0 Update 3q - VMware ESXi 7.0 Update 3q Release Notes - (WRITE SAME (16) commands with the unmap bit set fail on thick disks).