Does chrome.storage.managed policy work for self-hosted extensions?

I am trying to configure an extension using the method described at Manifest for storage areas . I'm pretty sure I have everything set up correctly, but I am not seeing the policy value in chrome://policy(it is shown as Not set) and, obviously, there is no policy seen from

chrome.storage.managed.get(null,(d)=>{console.log(d)});

I've checked my schema and the config file I uploaded in the Admin Panel at https://www.jsonschemavalidator.net and it seems to match. It's very simple, schema.json is

{
    "type" : "object",

    "properties" : {
        "PolicyTest" : {
            "type" : "string"
        }
    }
}

and in the json file

{
    "PolicyTest" : "test"
}

Before I spend a bunch of time debugging this, I thought I would quickly ask- could this be because the extension that I am configuring this for is not hosted on the Chrome Web Store? It's hosted myself using the method described at Autoupdating.

Other than that, I'm not really sure why this isn't working- the device running Chrome is Linux, although I have also checked on a managed Chromebook, and I've checked things like making sure I've selected the right OU, refreshing the policy, and so on.