no Suggestions for word constraints

In my search:suggest call I specify a word prefix constraint on a element. For some reason I am not getting suggestions for that prefix constraint search:

This code illustrates my problem:

let $doc := 
<doc>
  <title>Show me some suggestions!</title>
</doc>

return xdmp:document-insert('so.xml', $doc);

import module namespace search = "http://marklogic.com/appservices/search"
    at "/MarkLogic/appservices/search/search.xqy";

search:suggest('title:', 
  <options xmlns="http://marklogic.com/appservices/search">
    <constraint name="title">
      <word collation="http://marklogic.com/collation/en">
        <element name="title" />
      </word>
    </constraint>

    <default-suggestion-source>
      <word-lexicon collation="http://marklogic.com/collation/en"/>
    </default-suggestion-source>
  </options>
)

This does not result in any suggestions. Only after adding the following suggestions-source, I get the expected suggestions:

<suggestion-source ref="title">
   <word collation="http://marklogic.com/collation/en">
      <element name="title"/>
   </word>
</suggestion-source>

This does not seem to be the same with range or collection constraints, suggestions work out of the box and without a suggestion-source.

Is there a reason why suggestions are not shown for word constraints, e.g. performance? Is this behaviour documented somewhere?

Using MarkLogic 9.0-8.