ARM Template Parameters not recognised by DevOps Task in Override Parameters Dialog for ADF CD

How do I fix:

Warning: Failed to download the template parameters file from the given path '$(System.DefaultWorkingDirectory)/_myproject-poc/ArmTemplates/ARMTemplateParametersForFactory.json' with the following error: The item ArmTemplates/ARMTemplateParametersForFactory.json could not be found.

in the Parameters Override dialog, when deploying an ARM Template from files that were built in a pipeline?

in the Parameters Override dialog, when deploying an ARM Template from files that were built in a pipeline?

I've done this previously when my source artifact is a direct reference to a Git repo which contained the Template files, and it worked perfectly. ADO recognised the Git repo, found my target branch, found the relevant files within the codebase, and was able to parse the parameters file, to give me the UI to override those parameters. All good.

Now I'm trying to move to a model where a pipeline builds the Template files for me and publishes them as an artifact to use.

Artifacts generated contain below files:

The pipeline seems to work fine, and I've verified that all the relevant files are created.

But when I try to use those files as the artifact source, the Override Parameter dialog can't "find" them.

What am I doing wrong, and how do I fix this?