Works as intended. But thanks for raising the issue of missing documentation here!

When creating your theme with a custom layout, you can create a dynamic grid-template-layout. Check the following example and let me know if it makes sense:

const HIDEABLE_COLUMNS = 6;
export const getTheme = (hiddenColumns) => ({
 Table: `
 --data-table-library_grid-template-columns: 32px repeat(${HIDEABLE_COLUMNS - hiddenColumns.length}, minmax(0, 1fr)) 36px;
 `,
});

For every hideable but shown column, we allocate a minmax(0, 1fr) grid item. In the example the first and last column are not hideable and take up a fixed space of 36px. The rest of the displayed columns take up all the remaining space evenly.

I had the same issue, even after implementing what @rwieruch suggested.

Then I realised I had horizontalScroll: true as well. Once I removed it the new css is applied dynamically.

So posting here in case someone else has the same issue.