windows 2019 core build hangs during compact.bat

[ghost]

i created a custom build script with the following content

#!/bin/bash
packer build \
  --only=virtualbox-iso \
  --var vhv_enable=true \
  --var iso_url=~/downloads/17763.379.190312-0539.rs5_release_svc_refresh_SERVER_EVAL_x64FRE_en-us.iso \
  windows_2019_core.json

when running the build it seems to stop during the compact.bat execution

watching task manager within the vm it appears that both udefrag and sdelete have completed their executions.. but the build stops here.. this would appear to be the end of the provisioning.. but the post processing step does not appear to be happening..

perhaps my ignorance is showing.. but i am at a loss as to how to troubleshoot this further..

[ghost]

this issue is mentioned upstream..
joefitzgerald#280

downgrading packer to 1.3.5 has allowed me to work around the issue