Thanks for the bug report and all the info. It turns out the backup schedule is being set when the NextCloud account is created, but not when it’s later edited. We’ve committed a fix, which should go out with the next release, so I’ve marked this as “fixed”.
After the next release, if anyone still experiences this for newly created NextCloud accounts, it would be helpful to know.
In the meantime, for existing accounts where this is happening, going to the Settings > Accounts > Backup page and editing the backup schedule to something different to the current state should address the issue. After you’ve made a change you can then change it back to your preferred schedule (also on the Backup page), and it should then stick to that.