HEIF10 representation doesn't contain alpha channel

When using the heif10Representation and writeHEIF10Representation APIs of CIContext, the resulting image doesn’t contain an alpha channel. When using the heifRepresentation and writeHEIFRepresentation APIs, the alpha channel is properly preserved, i.e., the resulting HEIC will contain a urn:mpeg:hevc:2015:auxid:1 auxiliary image. This image is missing when exporting as HEIF10.

Is this a bug or is this intentional? If I understand the spec correctly, HEIF10 should be able to support alpha via auxiliary image (like HEIF8).