Unable to load CSS fonts from WKURLSchemeHandler in a https website

I am trying to cache fonts natively in a hybrid app, so that CSS hosted in an https website loaded in WKWebView through loadRequest can reference them like this for a performance boost:

    @font-face {
      font-family: 'MyFont';
      src: url('my-assets://Gordita-bold-subset.woff') format('woff');
      font-weight: normal;
      font-style: normal; }

The problem happens when I register a WKURLSchemeHandler for my-assets.

The handler never gets called and the Safari Web Insepctor shows this:

[blocked] The page at https://www.x.com/ requested insecure content from my-assets://Gordita-bold-subset.woff. This content was blocked and must be served over HTTPS.

Interestingly enough, if we try to serve content with <img src="my-assets://test.png" this restriction does not apply.

Are there any workarounds other than using the private API WKProcessPool._registerURLSchemeAsSecure?