Crazy this issue has persisted four months... I'm on a 2018 Mini with one LG 5k thunderbolt monitor, and one generic 4k HDMI-connected monitor. After upgrading to Big Sur, every window that used to restore onto the hdmi monitor is either half off the screen, or completely off as if the desktop were double or triple the width of the actual display. It happens every time my displays sleep throughout the day, so I have to move all my windows back every time I take a break from work.