I've been able to reproduce this several times so just for reference.
I'll fold an if() statement with brackets then add an old-style comment out before the if and after it's closing bracket. As soon as it changes color the + (plus) disappears and when it's un-commented again there is no + (plus). Even trying an unfold all doesn't work. The line numbers skip from where the fold was though. This is purely visual, you can save the code even it this state and it retains it. It just doesn't show it. Closing the window and reopening it retores it.
Using version 4.00.50.
