Browse to the edb file path and delete the folder there named something like 9726BCE3-622C-4675-8A20-57BE574A5C9B12.1.Single Remove all four directories that are located under the following folder: C:\Program Files\Microsoft\Exchange Server\V15\Bin\Search\Ceres\HostController\Data\Nodes\Fsis
cd C:\Program Files\Microsoft\Exchange Server\V15\Bin\Search\Ceres\Installer.\installconfig.ps1 -action I -datafolder "c:\Program Files\Microsoft\Exchange Server\V15\Bin\Search\Ceres\HostController\Data"
The search infrastructure is rebuilt in Exchange 2019 and this is by design.
Exchange 2019 uses the new search engine called "Big Funnel", using Bing technology to make the search even faster and provide better results. Search indexes are not longer stored on disk per Mailbox database, but search indexes are now stored inside the Mailbox database on a per Mailbox basis. So the context index for the database shows NotApplicable in Exchange 2019, and the unhealthy index state will become a thing of the past.
For reference: EXCHANGE 2019 RELEASED AND AVAILABLE ON MICROSOFT VOLUME LICENSE CENTER
The change is due them moving the Content Index State from being a Database level feature (where it has to be in sync as part of a DAG), to being at the mailbox level.