Matrix view: Notes with stipple

What does it mean when notes in the matrix view have a stipple visual 
effect on them?

I was experimenting with time signature changes in a song I'm writing. 
After doing that, notes in the bar I was working on suddenly took on a 
stippled appearance. That stippled look has remained even after deleting 
the tempo and time signature changes and saving and re-opening the file.

What does it mean? Why would changing the time signature of a bar have 
any kind of effect on the properties of notes?