This is another little one that’s been a little tough to figure out the repro for, but it boils down to this. If you set custom widths for the new columns, and an element within one of the smaller columns bumps up against the edges, it can cause the layout to break.
For example, in this screenshot you can see the column percentage values on the left, and how the button text within Column 2 is forced to wrap to a second line, and how that pushes Column 3 down a level.

If I remove some text from the button, the widths go back to lookin like what I expect.

(Side note: Is there a technical reason why we can’t write the values for these column widths the same way we could for the classic versions? For example, 1/5 + 2/5 + 2/5, etc.? I’m just so used to the old way, it took me a minute to figure out why the new columns weren’t responding when I used fractions)