You know, I am not quite sure, I just pick one of the custom template.
I'm guessing that has to do with the parameter acting weird?
If you had an LP inside an LP then you would be forced to use the referrer. Normally this would not work, nor be necessary.
I worked on a similar project recently and these two pages helped me:
For the landing page: https://jennamolby.com/how-to-display-dynamic-content-on-a-page-using-url-parameters/