This is easier to look at when you write /leavesublv/gvsv( "bar' ) vs /leavesublv/leave/gvsv( 'baz' ). It appears to be a bug where the return of $bar isn't generating a leave. Both conditions should trigger the warning as long as leave doesn't notice that its next op is going to be a leavesublv. So two bugs - one is that you didn't get the warning enough and one that leave isn't aware of leavesublv.

It also looks like the fix for this makes the common case ever so slightly slower.