Hi Laurent.
This is not SPPB issue, but an issue with support of background-attachment: fixed; on devices with iOS. Parallax used in SPPB is based on this property. Therefore, you can disable this for all mobile devices using CSS Media Queries or only for iOS devices using CSS Media Queries for iOS.
Something like
@media (max-width: 991.98px) {
.your-section-class {
background-attachment: scroll !important;
background-position: center !important;
}
}
Or looking for third-party parallax plugins with iOS support.
But in general, any parallax effects on mobile devices are redundant and lead to poor page speed.