sharepoint online – Rest api caml query not returning field in ViewFields

I have a CAML query that I am running through the REST api, the simple example below requests 2 fields – FileLeafRef and CustomField. Custom field is simple a text field, nothing special.

CustomField is returned whereas FileLeafRef is not.

If I run the same CAML via CSOM or the old WCF based services then FileLeafRef is returned…

<View Scope="RecursiveAll">
  <Query>
    <Where>
      <Eq>
        <FieldRef Name="FileRef" />
        <Value Type="Text">/SomeSite/SomeDocumentLibrary/SomeFile.txt</Value>
      </Eq>
    </Where>
  </Query>
  <RowLimit>1</RowLimit>
  <ViewFields>
    <FieldRef Name="FileLeafRef" />
    <FieldRef Name="CustomField" />
  </ViewFields>
</View>

I know I can use a $select querystring, but this is on an app that the user can specify any number of columns to retrieve and querystrings have a limit.

So is there something I am missing? Or does REST CAML act differently to the other two…

N.b this is against SharePoint Online