Your Hamamelis 'Diane' is the scion grafted onto a rootstock of a (possibly wild) species which has sent up a sucker from said rootstock which is genetically different from the scion 'Diane' and that's why the flowers (on said sucker) are yellow.
This is a 'problem' as such in that this will be an ongoing maintenance chore for you to cut all emerging rootstock suckers because they are more vigorous than the graft.
