Search for file names doesn't work as expected

Since full text search 0.8.0, it should be possible to search for file names by using "in:filename" in the search bar (see here: https://help.nextcloud.com/t/full-text-search-0-8-0-better-integration-bugfixes/32946).

Unfortunately, this feature seems to work only under certain circumstances:
I set up fulltext search and created the initial index with sudo -u www-data php ./occ fulltextsearch:index.
Now when searching with "in:filename", the search is not able to find the files contained in the initial index.
Only when adding new files (and after waiting for the cron job to run), these new added files can be found with "in:filename".

There are no errors shown, not even in the Nextcloud log file.

I also tried to reset the index and creating it from scratch.

Am I missing something?

Nextcloud 13.0.6
Full text search 0.99.3
Full text search - Elasticsearch Platform 0.99.2
Full text search - Files 0.99.4
Elasticsearch: 6.4

Start with the occ fulltextsearch:index entry point and compare its initial-index path with the cron-triggered indexing path. Reproduce the issue using in:filename after creating the initial index, then verify that initially indexed files are searchable without requiring newly added files or showing errors.