You mean when you upload/change files via the official Nextcloud Windows client, those changes are not visible in official Android and iOS apps? Then the issue is on a very different level which I suggest to investigate instead of applying such a workaround (which might not work for the same reason). Are uploads/changes done via Windows client visible on the server e.g. when accessing the web interface?
For the cron job, just to check whether they run now successfully:
journalctl -u cron
Do you still see (CRON) info (No MTA installed, discarding output) output for recent calls (which would indicate an error ourput)?
Bear with us that we cannot maintain the same full CLI and web UI extensions to every software title which single-purpose OS/software projects do. If Nextcloud is all you want to use, NextcloudPi is a great choice, and I’m happy to see that it finally supports Debian/Raspbian Bullseye with a new maintainer. For the cron jobs, it will do nothing more than adding them via crontab -u www-data -e or using /etc/cron.d 
.