Iphone 7 + css fixed position issue

I Put some sticky form on some sites and I have an issue with the focus on field just with Iphone 7

I try many browser and many phone (iphone 4s, 7, galaxy, alcatel …) my only problem is with iphone 7 and on those specific form fixed on bottom of the window.

If I click on a field it doesnt focus on it, if I click on first field its working, but if I click on the “tab next field button” It focusing way farder then the next sibbling field.

There are two of those website :
https://www.westburymontreal.com/
https://lestourssaintmartin.ca/

Do you have any clue ?

The page I need help with: [log in to see the link]