Just posting to help others that face this issue with the thank you message changes (done in Forms) not updating in your Landing Pages. Basically all you have to do is unpublish the landing page and republish it (the old IT adage of “turn it off and on again”). It is not a cache issue and will not resolve itself by deleting your browser cache, cookies and history. If unpublishing and republishing the landing page doesn’t work, clone the form that it uses and change this in the landing page settings.