Not a lot of time to pinpoint the precise reason, but as long as you understand the principle behind the issue you’re facing, I hope you’re able to find the issue by yourself.
Basically, Lenis is loading at a time that your page is of height X. But once you use the page, scroll down and/or more things/images are loaded, the page grows in size (X+200px for example).
This is the cause of the issue. I hope that with that you can find what’s causing it.