https://drafts.csswg.org/css-grid/#subgrid-item-contribution

In this process, the sum of the subgrid’s margin, padding, scrollbar gutter, and border at each edge

https://drafts.csswg.org/css-grid/#subgrid-edge-placeholders

For each edge of a non-empty subgrid, to account for the subgrid’s margin/border/padding at that edge [...]

The 2nd quote doesn't include the scrollbar gutter, but it should, like the 1st one. As per #9815, browsers are including it.