Keeps unstacking when trying to create a single stack of windows in space on external monitor

I can create a single stack of windows on my internal/built-in screen, however when I try to create a single stack of windows on my external monitor, it will unstack itself immediately (it won't stack).
If I split the screen in half with two windows, I can still create a stack for each window (2 separate stacks in the same space).

skhd -V doesn't show any logs when attempting create a single stack of windows.
I have tried restarting the laptop, and restart both yabai and skhd.

Versions:
macOS 14.1.1 (this was happening in macOS 14.1 as well)
yabai v6.0.1
skhd v0.3.9

https://github.com/koekeishiya/yabai/assets/31675641/17a305f9-6c21-45b1-a22e-1978ab280748

No files, tests, or entry points are named. Start by reproducing the single-stack failure on macOS 14.1.1 with yabai v6.0.1 and skhd v0.3.9, comparing internal and external monitors. Done means a single stack remains stacked on the external monitor without disrupting the working split-screen case.

Written by the indexing model from the issue text.