A link explaining how I realized this: https://bbs.archlinux.org/viewtopic.php?id=268786
If you have an unexplained janky behavior, check the panels that your Plasma has in reserve - it saves the settings for monitors you've previously had set up, even if they're disconnected.
Go to Edit Mode > Panels and Desktop Management and see what's going on in there, and maybe delete old desktops if they're causing problems.