It seems the top 50 square pixels of tiles 19 to 23 don't actually update/change/cycle (or whatever you call it) - these are the firey pit tiles, and the top left is the left corner of the pit. Is this deliberate or a bug? Is there a defined limit on the delay between these tiles updating or does it depend upon the speed of the computer, memory etc?

Please let me know as I want to replace these tiles with another set, but not having them all change is preventing this.