This is one of the challenges to SharePoint search content on a list from multiline field. Multi line text field cannot be indexed as it’s an Unsupported Column Type.
Indexing the field is separate from search crawling which is needed for search result. Make sure the column has a managed property which allows a column to be searchable, retrievable and quarriable.
I noticed an issue on a list with over 10000 lists items where content for multiline text was not working on list search page and not showing the required outcome whereas it shows the required result on the site search. So I believe that in a SharePoint list, multiple line content search do not working properly if list view threshold is over 5000 items.
Steps to Create Managed Property
First you need to create a site column and then create a Managed Property
1 – First create a site column for multi-line text field by going to site settings –> Web Designer Gallery –> Site Columns –> Create
2 – Create the site column and add it to a content type or SharePoint list.
3 – Add atleast one list item to the list, its important so it can create the crawled property for the body column. Now run full crawl on the list.
4 – Now go back to Site Settings –> Site Collection Administration –> Search Schema and click on New Managed Property.
5 – Now select the add mapping properties
6 – Leave “Text” selected, also check Searchable, Queryable and Retrievable.
7 – You can map multiple properties against this managed property to make sure you are fetching all the relevant columns.
8 – The crawled property is different depending upon what you named it. My column was called Body, so I found it under “ows_body”, “Body”, “ows_Body1” and “ows_Body_1”. I added multiple properties because I used body field in multiple lists.
9 – Now you can start the list index or wait for some time so search crawl can run automatically.