I'm experiencing the same problem OK, I have been running a game for about 3 weeks now and everything was fine. Now I am having problems in the sense that the fog of war won´t reveal parts of my map. I have a large map that required me to magnify to 150, just so I can see the tokens (map was large). I decided to crop the map so that I can make it a big bigger and when I do, I can no longer reveal the edges of the map. Certain parts will reveal and others will stay highlighted (just prior to the reveal, but it will stay like that but won´t reveal. All this started right after the last update.
Currently using Firefox 12 and windows vista.
I experienced this same problem last night. I don't have a huge map or anything, but I am using a zoom setting other than 100%. If I want to reveal an area on the edge of the map, I have to start my box far away from the edge and then end the box close to the edge for it to work. If the bounding box is entirely drawn too close to the edge, then I get the behavior Hector described. I'm in Chrome.