SychO Ah, I found the issue. Updating the cron job like this fixed the problem I was having with NotificationSyncer:
sudo -u huseyinfiliz /opt/plesk/php/8.2/bin/php flarum schedule:run
Honestly, I still don’t fully understand the reason, but since a fresh Flarum installation didn’t have the issue, I tried everything I could think of—and this somehow fixed it.
I guess it was a permission or server configuration issue.
Thanks for your support!