"3 Column Middle" acting like "row"

[rtphokie]

I've installed Amethyst on two M3 MacBook pros, each running Sequoia 15.0, on an UltraWide (5120x1440) monitor. It works great on one, but is acting unexpectedly on the other in "3 Column Middle" mode. On the misbehaving one:

• the first window comes up full screen, as expected (though I wish it would open in the middle only)
• the next window brought also takes up an entire row, one row per window, instead of the expected middle position with the previous window to the left (or right)
• swapping windows with the mouse doesn't work

Interestingly both machines behave identically in "4 column [left|right]"

Amethyst prefs are identical between both laptops and look like this.

General

• Ignore menu bars
• Send new window to main pane
• Follow thrown windows between spaces
• Hide menu bar iconds
• window count, width, height, etc. left at 0 default
• Window Margins
• Smart Window Margins
• screen padding left at 0 default
• Disable screen padding on built-in display
• Heads Up Display: Display layout when changing layouts
• Heads Up Display: Display layout when changing spaces

Layouts

• 3Column Middle
• 4Column Right

Mouse

• Mouse follows focused windows
• Focus follows mouse
• Swap windows using mouse
• Resize windows using mouse

Floating

• Float small windows
• Automatically float applications listed (empty list)

Debug

• Show debug info: About layout
• Updates: get development builds

Each is configured through the app (not using a amethyst.yml file). I've also verified Automatically rearrange Spaces based on most recent use is not selected in Mission Control System Prefs. and that Amethyst is enabled in System Prefs -> Privacy and Security -> Accessibility. I've also restarted Amethyst several times to ensure any updates to prefs are loaded.=

Any ideas? What have I missed? Is there some MacOS setting I've got different between the two that is causing this problem? Is there a way I can export the config of the behaving laptop's installation as a YAML file for use on the misbehaving one?