Marketo Form onSuccess no longer leading to desired follow up page

I have a very similar issue to this problem: https://nation.marketo.com/t5/product-discussions/onsuccess-handler-to-follow-up-page-has-stopped-working/m-p/326068#M184055
In this discussion, it is said that a script from GTM is causing the issue but when I prevent my GTM from loading on the site, it does not fix my problem and the onSuccess function still doesn't lead to the correct page. It worked properly for a very long time and today, it randomly stopped routing leads to the correct follow up page. Any help is appreciated.