Here, I defined the return object as an entity called "test" which is specified as value of key with type "entity".
<property key="test" type="entity" required="false" isPath="no">
<caption>a</caption>
<category>x</category>
<description/>
</property>
<property key="myKey" type="microflow" required="false">
<caption>b</caption>
<category>x</category>
<description/>
<returnType type="Object" isList="true" entityProperty="test"/>
</property>
Did you specify the modulename of the entity? Like "MyFirstModule.Entity"