Hi hamsterPL,
As shmit suggested, this is expected behavior. When you set the debug lines for 999, they will remain on screen for that duration even after you create a second instance of the lines where you changed their duration to 1 second, as you did in your example. When you close the level that ends the duration and it is expected that the lines be gone when you reopen the level. If you were to reopened the level and the lines were still there, that would be a bug.
As a test I set multiple instances with various durations 1, 10, 20, 30 seconds and they all stayed on screen for the appropriate amount of time.