After updating plasma-workspace and others to 5.7.4 following yesterday's uploads to sid, this issue appears to have magically disappeared like I suspected it would do. Hooray!
That said, I noticed that now when the panel is locked, unchecking items in General -> Extra Items in the tray config doesn't do anything until plasmashell is restarted (see also https://bugs.kde.org/show_bug.cgi?id=365618 upstream). -- Regards Ignacio R. Morelle