Steps to Reproduce:

• Have $wgPageFormsUseDisplayTitle = true
• Run a pfautocomplete API query (a`pi.php?action=pfautocomplete&format=json&concept=example&substr=Ex`)

Actual Results:

• Empty result set returned

Expected Results:

• For pages that have the Display_title_of set, this value should be used for the substring comparison
• For pages that do not have the Display_title_of set, the title should be used

Generally, the issue seems to originate from the fact that when $wgPageFormsUseDisplayTitle = true PageForms expects SMW to set Display_title_of for every single page. Pages that do not have that property set are ignored. However, SMW only does so when {{DISPLAYTITLE:title to be displayed}} is set.