When creating a store, if you add more than 11 parent categories in one go (using the + button) only 11 are created - the first ten defined and the last one defined, those in between just vanish.
When adding new parent categories to existing ones after some pre-existing parent categories have been deleted, the new ones seem to get inserted in random positions. Iy would seem more logical for them to be positioned at the end.
When adding new parent categories to existing ones there is no error if the maximum of 30 is exceeded - some are just ignored.