Which version of android are you running as earlier versions don’t support fixed positioning.
Its one of those things that we can’t debug without having the same device as you. I would suggest that you you try and make a simplified version of the site and then see if it works in the base version. It could take a while to find the cause or you might just stumble on to the answer straight away.
Try disabling js and see if that makes a difference initially.
Alternatively delete one section at a time and re-test and see if the problem is still there each time. It’s likely to be a matter of trial and error.
I would expect complex parallax sites like that not to work on mobile or tablets very well anyway.