Just out of curiosity, did you exit your browser completely and then rejoin the game to see if that alone corrected it?
I'm wondering if it could be a memory issue. The Cerato map is huge, so its requirements are going to be much more significant. Applying an update forces the browser to restart, clearing its memory usage and starting with less overhead.
The attack overlay is currently generated as one large image. I may need to break that up into smaller pieces on the large map size.