Hi,
This is caused by the Read-Only settings of the Dataview.
It is not properly inherited by the snippet.
Might be a Mendix bug ;)
Does the page parameter change the form layout from horizontal to vertical?
In a dataview, you can specify whether you want the Form Orientation horizontal or vertical, such that the input fields have their labels either next to it or above it. However, in a snippet you can't set the Form Orientation and it's always set to vertical (see screenshot below). As you mention correctly, it's not just a CSS class, but a complete different rendering of HTML.
So, what to do?