This topic is about blocks, so I thought you're talking about it.
What you want is moving the meta boxes to the sidebar. That can be done with the same parameter 'context' => 'side' for meta boxes. See this docs.
Previously, Gutenberg moves all the meta boxes to the "after content" position, which is what you're seeing. It probably changed its behavior to accept side context. I've updated the Builder to reflect the change, so you can see the option within the Builder.