Please doublecheck in the page settings that you have 'Only Update on Drop' checked. Short of that, dynamic lighting seems to be the last layer loaded so those with slow connections will see the map before darkness cloaks things properly. The usual area of concern is when entering a map (or reloading the game tab), and while you can use fog of war to manually cloak the whole map (aside from a landing area) and then drop the fog for entering a map for the first time each session that doesn't help with mid-session accidental reveals. Using fog of war in conjunction with your dynamic lighting and manually unfogging area by area, and making sure all monsters live on the GM layer when not in use are possible technical workarounds. I'm sorry I don't have a better answer for you at this time. :(