Objects are not Growing What placed in a Header or Footer
Some of the Header and Footer types are fixed size, and do not support growable objects.
Page Headers and Footers, Continuation Headers and Footers, and Repeating Headers and Footers are fixed size in order to keep page layout deterministic.
Objects like memos and linked reports generally have the Growable property set because they are not fixed size. If you place a 'Growable' object in a section that is fixed size, the Growable property is automatically disabled.
If you need the header to contain Growable content, consider creating a Group and using a Group Header or Group Footer instead.