user names are not important in *ix system - UID and GID are important. you need 33:33 by default you don’t need a user www-data you need a user with UID 33 - or you need adjust your container user as described the guide above. I can’t tell if unraid is doing some additional voodoo which you have to understand yourself… maybe they adjust container user to be “nobody” - this out of scope of this forum.
there is a lengthy post about cron and unraid Unraid, Nextcloud, and what the hell is a Cron? maybe it helps you to understand your problem but I’m too lazy to follow all the references…