Error: Index does not exist. (No such file or directory)

I recently ran into an issue with the new feature in Domino 12 to exclude fields from full-text search indexes.

You must enable this feature by adding the notes.ini setting FT_DEBUG_DISABLE_DB_RULES=0 to the Domino server AND restart the Domino server.

Navigate to Application Properties -> Full Text Index and select the items to exclude fron the FT index.

You can then delete an existing FT Index and create a new one. The size should be smaller now. So far so good.

If you now do a search either from the Notes Client or progammatically ( LS or Java ) you will get an error.

Y03/26/2022 05:43:54 AM  AMgr: Start executing agent 'qry' in 'ftdemo.nsf'
03/26/2022 05:43:54 AM  Agent Manager: Agent printing: initial query.
03/26/2022 05:43:54 AM  Full Text message: Index does not exist. (No such file or directory)
03/26/2022 05:43:54 AM  GTR search error for "/local/notesdata/ftdemo.ft/ftgi": Index does not exist.: Full text index not found for this database
03/26/2022 05:43:54 AM  Agent Manager: Agent  error: NotesException: Domino Query execution error: 
Full text index not found for this database -  error during planning and tree generation  


idNumber > 20 AND idNumber < 30

	(Call hint: FTSearchExt, Core call #0)


******************

03/26/2022 05:43:54 AM  Agent Manager: Agent  error: 	at lotus.domino.local.DominoQuery.execute(Unknown Source)
03/26/2022 05:43:54 AM  Agent Manager: Agent  error: 	at JavaAgent.NotesMain(JavaAgent.java:35)
03/26/2022 05:43:54 AM  Agent Manager: Agent  error: 	at lotus.domino.AgentBase.runNotes(Unknown Source)
03/26/2022 05:43:54 AM  Agent Manager: Agent  error: 	at lotus.domino.NotesThread.run(Unknown Source)
03/26/2022 05:43:54 AM  AMgr: Agent 'qry' in 'ftdemo.nsf' completed execution

The issue is reproducible on Windows and Linux. I have opened a support case with HCL.

If you disable the feature, restart the server and recreate the index, everthing works good.