Hi L,
Thank you for your message and sorry for the inconvenience you are facing.
The behavior you are seeing is due to the template. When using SP Page Builder, the page is rendered inside the site template layout, and the overall container behavior (boxed vs. fluid) is controlled by the template rather than SP Page Builder itself. This is why it works differently with Helix compared to Cassiopeia.
To adjust this in Cassiopeia, please enable the "Layout Fluid" option in the template settings. For reference, here is the location: https://files.fm/u/tkz9h5u2w2. Enabling this generally allows full-width sections similar to what you get in Helix.
If the result still does not match your requirements, you may need further customization on the Cassiopeia side, as it is a Joomla core template and not developed or maintained by us. In that case, we recommend checking or requesting additional suggestions in the official Joomla forum, as Cassiopeia behavior falls outside our support scope.
Best regards