It is in a standard library, the element's called Audience seating. But the actual problem is much worse because you can see stair lines overlapping and manually fixing display order does not help at all.
It feels like it happens when the project gets larger and I guess it just become harder to handle a hierarchy of all the lines for Archicad, because when I started the project from the beginning the problem was gone.
But then later in the same project the same issue started happening again but now it's with happening with beams. Why can't just lines that are cut be over all the other lines?
So what i did was i copied the beams that caused the problems.
Then i removed the old beams that do not display correctly
And moved the beams that were copied to the right place.
That solved the problem for me. So I think what's going on is when you copy some object it forces it to be displayed on top of all the other objects and you can't really change its order with this buttons because i tried multiple times and it just didn't work.
But i would really like if archicad did not mess the order of lines every time i copy some object. Can you please look into this issue