I have been having this problem since 12.1 and I just upgraded to 12.5 hoping it would fix it but it did not.
The Root issue is that the Search String is completely broken, if anything is entered into it no results are returned. It is as if there are missing indexes that are required.
Here is a video that shows I am looking for a PO number in the search, but nothing comes back. I happen to know the date the po was added so I can filter it to find the ticket but that way more work.
Here is the video.
https://www.dropbox.com/s/6fktb98ojjqbo62/ST15x_Search_Defect.mov?dl=0
Can someone from smarter tools please reply with a solution to this issue. Very Frustrating.