When you reopening the map, all prefab actors will be reverted to saved prefab version, that unfortunately will also invalidate actors' lightmap data.
It's not ideal, so in next update, only those whose prefab has been changed will be touched when reopening the level.
For select parent prefab, currently you can use "Shift+Alt+S" hotkey (or through right click context menu). I plan to add "group" feature in later updates.