Continuous view: Key change forces double barline
Create a score and look at it in continuous view. Insert a key change (either global or local) and a double barline appears before it.
When you switch to page view, the barline is normal.
As a workaround you can insert a new single barline after you change the key and it will be displayed properly.
I haven't tested previous version for regressions, but I wouldn't hold up release of 3.1 over it.