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):
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