With the writing-mode property value set to vertical-rl the start of the element in the block direction is changed from top to right, and this also affects the scroll-margin-block-start property.
In this example, snap position is set to start in block direction, which now is on the right side because of the writing-mode value. Scroll-margin-block-start is also on the right side because of writing-mode value. so the element in focus should snap into place with a 20px distance to the top of the container. But instead, because of the writing-mode value, the element in focus is offset by a 20px margin between child element and container on the right side.