Workaround : If you add advanced filter and the record count comes below 500 you will have the inline search option for those text area.
For more information please follow the below document.
https://support.gainsight.com/Reports_and_Dashboards/User_Guides/Customers_Tab%3A_Your_Book_of_Busines
Customer's request : Rather than using Advanced filter, remove the limit of record count in Custom Tabs so that the inline search in text area is enabled.
Note: Pic attached.