Nightly auto-upgrades are great, except when they aren’t. In this case, going from app-misc/screen-4.0.3-r7 to app-misc/screen-4.2.1-r2 changed the default SCREENDIR from /run/screen to /tmp/screen. Normally wouldn’t care, but I had running screen sessions and couldn’t get in; oops!
Only realized about the SCREENDIR variable after I’d already downgraded back to 4.0.3-r7.