We just came across a bug that Support was able to reproduce - and thought I'd alert the community as well. Basically, if you have an existing Marketo LP (with a form) and confirmation LP, and you change the URL of the page(s) using "URL Tools > Edit URL Settings" after the pages have already been approved, the form-submit will not take you to the confirmation/thank-you page, and instead take you to your fallback page for your instance (usually your website's homepage).
Here are the steps to reproduce this (note, we're using guided LPs here):
- Create a Marketo LP and insert a form onto the page
- Create a Marketo LP for the confirmation page
- Use this confirmation LP as the thank-you page (defined on the main landing page using "follow-up type: Landing page")
- Make sure both page are approved
- Test that the page and confirmation page are working properly by accessing the main LP and submitting the form.
- Once confirmed, use "URL Tools" to change the URL of both pages (under "URL Settings"):
- Test the LP again. It will not redirect to the thank-you page as defined within the form.
Until this is fixed, the only way to resolve this is to delete the form from the main LP, re-add it to the page and re-define the "follow-up page" once again.
Ugh.. that's definitely frustrating. It didn't used to do that, so it's a bummer that some update has caused this 
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