I am using AEM 6.2 . Issue is in QAT and in higher environments. I did check my local and its working as expected.
<text
jcr:primaryType="nt:unstructured"
sling:resourceType="cq/gui/components/authoring/dialog/richtext"
name="./text"
fieldLabel="Headline"
useFixedInlineToolbar="{Boolean}true">
<rtePlugins jcr:primaryType="nt:unstructured">
<styles
jcr:primaryType="nt:unstructured"
features="*">
<styles jcr:primaryType="cq:WidgetCollection">
<emphasis
jcr:primaryType="nt:unstructured"
cssName="emphasis"
text="Emphasis"/>
</styles>
</styles>
</rtePlugins>
<uiSettings jcr:primaryType="nt:unstructured">
<cui jcr:primaryType="nt:unstructured">
<inline
jcr:primaryType="nt:unstructured"
toolbar="[styles#emphasis]">
<popovers jcr:primaryType="nt:unstructured">
<styles
jcr:primaryType="nt:unstructured"
items="[styles#emphasis]"
ref="styles"/>
Please let me know if the style properties are not defined properly.