Subject schema not showing up as dropdown in Attributes panel

I have created a subject schema; here is the sample code: 

<subjectScheme>
<title>Subject Schema Definition</title>

<!-- 1) Permissions (for prolog@view) -->
<subjectdef keys="permissions">
<subjectdef keys="perm_internalonly">
<topicmeta>
<navtitle>InternalOnly</navtitle>
<shortdesc>Only visible to internal staff</shortdesc>
</topicmeta>
</subjectdef>
<subjectdef keys="perm_lix_experiment">
<topicmeta>
<navtitle>LIX_experiment</navtitle>
<shortdesc>Experimental LIX audience</shortdesc>
</topicmeta>
</subjectdef>
<subjectdef keys="perm_lix_control">
<topicmeta>
<navtitle>LIX_control</navtitle>
<shortdesc>Control group for LIX experiments</shortdesc>
</topicmeta>
</subjectdef>
</subjectdef>
<enumerationdef><!-- bind to prolog@view -->
<elementdef name="prolog">
</elementdef>
<attributedef name="view">
</attributedef>
<!-- point to the subject group defined above -->
<subjectdef keyref="permissions">
</subjectdef>
</enumerationdef>

<!-- 2) Platform (for metadata@platform) -->
<subjectdef keys="platforms">
<subjectdef keys="platform_mobile">
<topicmeta>
<navtitle>Mobile</navtitle>
<shortdesc>Mobile platforms</shortdesc>
</topicmeta>
</subjectdef>
<subjectdef keys="platform_desktop">
<topicmeta>
<navtitle>Desktop</navtitle>
<shortdesc>Desktop platforms</shortdesc>
</topicmeta>
</subjectdef>
<subjectdef keys="platform_web">
<topicmeta>
<navtitle>Web</navtitle>
<shortdesc>Web platform</shortdesc>
</topicmeta>
</subjectdef>
</subjectdef>
<enumerationdef>
<elementdef name="metadata">
</elementdef>
<attributedef name="platform">
</attributedef>
<subjectdef keyref="platforms">
</subjectdef>
</subjectScheme>

The subject schema shows up in the Subject Schema panel in AEM Guides Web Editor. And these can be applied using drag-and-drop functionality. I do not get any errors when the subject schema is applied and saved. However, the values of the attributes are not getting bound by the subject schema. In other words, the values of the attributes in the Attributes panel are not showing up as drop-down values. 

Any input on how I can get the values of the attributes defiined in the subject schema show up as drop-down in the Attributes Panel? 

@divrajsingh or anyone on group, your guidance is much appreicated. 

I am testing this with latest AEM cloud SDK package.