Created attachment 4382
Screenshot with related setting displayed
The setting controlled by the slider in Window Manager Tweaks -> Placement (tab) -> "Minimum side of windows to trigger smart placement" isn't initially respected. Adjusting the slider to any value (even if it's the same value), applies the setting. See attached for a screenshot of the related setting.
On Arch Linux, if it matters.
Interesting, this seems to be working on my system all-of-a-sudden! I'm marking this ticket as works for me :D