Water has come down through that tube. It has rotted the fiberboard and caused the paint to flake off.
The tube must get water very intermittently or you would see it. Probably it was cut off during a renovation and the new ceiling put up.
I am curious where that tube goes, of course. If you can go up above your flat maybe you can find out. Maybe it goes up through the roof and gets a little water when it rains. You could try running a long plastic stick or tape measure through; leave it run up through and then when you go up there it will be easier to find the tube.
Ideally you find the top side and cap it. If you just plug your bottom side it will sit filled with water and might eventually corrode its way out.