Related to the message you got via email see this github issue: https://github.com/osTicket/osTicket-1.8/issues/1532
Besides that you should know that there is a limit for minimum 3 chars when a search is performed. As I can see on your screenshots your ticket numbers are only 2 number based, so you should consider changing it when possible. Btw. here at our osTicket with the default random 6 digit ticket number it works fine ;)
Additionally let me point you to the following pull request which states that the developers are working to improve the search: https://github.com/osTicket/osTicket-1.8/pull/1528