Had raise this issue via SMP and got the following reply:
the AppCacheBuster is not supported for HCP HTML5 applications out-of-the-box. The documentation is unfortunately misleading since it doesn't reflect the new HTML5 apps there and is based on the state for the Java applications:
For UI5 applications running on Java as well as for UI5 applications running on ABAP this file is generated automatically.
You should either remove the option data-sap-ui-appCacheBuster="./" on the bootstrap script which disables the application cachebuster or you create this file manually - but for
that scenario you have to take care that you handle the timestamps for the resources properly.
Regarding the HTML5 apps roadmap - till now it is not planned to support it.