I am new here so I can not post a comment to get more information. I do have Elementor Pro, but the container layout settings are also available on the free version of elementor.

It sounds to me like your container may be embedded inside another container. If you have set the container (flexbox or grid) to full width (100%), it will display the width to the max allowed by the parent container.

For example, if I have 3 containers (all 33% width, arranged horizontally) embedded in another container (100% width), the 3 will be evenly spaced across the entire width of the window. However, if I change the parent container to 50% width, then the 3 will be spaced evenly on the half of the window display. This also would affect the centering - it would be centered within the parent container not the full display.

You do not need custom CSS to set these layout properties. If this isn't the problem, please respond with more information.