As Richard Hipp from the SQLite forum has told me:

You cannot use a WITHOUT ROWID table as an external content table. The external content table must be a rowid table. The "content_rowid=" argument to FTS5 must refer to a column of type INTEGER PRIMARY KEY. If you make the "content_rowid=" refer to a TEXT PRIMARY KEY column, it won't work.

Removing the WITHOUT ROWID and content_rowid=path fixes the issue.