UI problem: Buttons disappearing

I have a level selection menu that contains buttons for selecting a level.
The buttons are programmically created.
If there are more than 13 levels, the buttons disappear:
13: i.imgur.com/6O9OZuq.png
14: i.imgur.com/s1pvnCg.png

Even if I simplify the Button creation code to bare minimum it still happens:

auto levelButton = levels_container->CreateChild<Button>();

The container of the buttons is a ScrollView.

Anyone knows why it might happen?