Thanks all for the input! My knowledge of HTML/CSS is basic at most, so I appreciate everyone's feedback in helping me understand how it all works.
The form was placed on the landing page using the Drag & Drop editor. I had a browse through the Community and read from other posts that doing this causes the form to be position:absolute, therefore creating this issue during a window resize.
What's the best way to work around this? Would it be a better option to place the form HTML directly into the landing page template? If so, how do I obtain the form HTML?
I also tried the iframe method but this got rid of my customized Submit button completely. Clicking on the default submit button also caused the follow up page to appear INSIDE the iframe.