SharePoint online document tile property in Search not appearing

I used Search rest end point to get the search results. However i observed an inconsistency between the Title property returned in the search results vs the actual document Title.

I found there is some mechanism based on which this property is populated and sometimes it could be document Title and sometimes the content of the document. To get the document Title, i created a new managed property with name ItemTitle and mapped the crawled property ows_title to this. Have also re indexed the site but the ItemTitle property is still null in the search results.

Can someone please help me to understand why i am not getting the value populated in the ItemTitle managed property.