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.
Would you mind trying to clear your cache following those steps, as well as trying with another browser and with an incognito window?
If this is still happening, I would recommend connecting with HubSpot Technical Support, as Support is included in your subscription and they will be able to provide real-time assistance for this matter, where you are able to share screenshots and specific examples here.
Hi, I've tested the cache theory across multiple laptops and browsers and unfortunately this isn't the issue. It's not even an internet propagation issue, as our form updates were done over 24hrs before going live with the email. That said, the landing page was already published, so I'm now looking to unpublish and republish the form and the landing page, but this is not a good thing to do while live!