To my surprise, the layout in the second (lower) appender above had the "myprop" property set to the value from the first appender's layout. By the way, both layouts are the same type. It's a custom layout I wrote.
Thankfully, if I set the property in both layouts to a different value, each layout has the correct value. So the problem seems to be that if a layout takes a parameter and you don't supply it you might end up getting the value set based on it being set in a previous layout.