I'm on macOS, using the current version of TB, 9.0.239, and have assigned the keyboard shortcut Shift-Command-D to the "Rotate Splitter" command.
Attachments are shown as list.
When I open an existing brain, the layout is by default "horizontal". Left side: plex; right side top: Notes, bottom: attachments list.
With a click on that tiny little "Rotate Splitter" button somewhere near the lower border, I get my preferred layout, "vertical": Top: plex, bottom left: notes, right: attachments list. So far so good.
But you have to aim really good to hit that microscopical "Rotate Splitter" button. So, I've assigned a keyboard shortcut to the "Rotate Splitter" function.
BUT: when I use the shortcut instead of the button, the layout change is WRONG, when being used just after having opened a brain. I get: upper half: plex, lower half, top: notes, bottom: attachments list.
Small detail: when I used the button to rotate to "vertical", and than use the shortcut to rotate back to "horizontal", the layout is wrong, too: left half: plex, right half: left: notes, right: attachments list.