If you change the file (or add a completely new file) on Android, and then manually trigger a scan of the folder, does Syncthing detect the changes? If not, then this may be a permission issue, where Syncthing cannot access the files on Android (maybe similar to https://forum.syncthing.net/t/not-all-files-getting-scannned-synced/16365?)
Another possibility could be that your KeePass application on Android doesn’t update the mtime of the database file. If that is the case, then Syncthing will not be able to detect changes in such a file. You may want to try using a different KeePass app to verify this.