TYPO3 Core

I have a backend layout configuration like the following:

2 {
    title = Default
    config {
        backend_layout {
            colCount = 4
            rowCount = 3
            rows {
                1 {
                    columns {
                        1 {
                            name = Startinhalt
                            colPos = 0
                            colspan = 4
                        }
                    }
                }
                2 {
                    columns {
                        1 {
                            name = linke Spalte
                            colPos = 99
                            colspan = 1
                        }
                        2 {
                            name = Seiteninhalt
                            colPos = 1
                            colspan = 3
                        }
                    }
                }
                3 {
                    columns {
                        1 {
                            name = Seiteninhalt
                            colPos = 2
                            colspan = 4
                        }
                    }
                }
            }
        }
    }
}

When I look at the layout in the backend, the columns beside each other always have the same size.

I've checked an installation with v11 and found one difference in the table properties.
In v11 the table has width="100%", in v12 the property is removed.

When I add it back to the table, the result is as expected.