In Salesforce the pre header is a field, similar to the subject line, as opposed to being part of the HTML, so when you have your pre header as part of the HTML and export to Salesforce it does not appear in the pre-header field.
We can turn on a feature in your account which will give you a pre header field that will appear in Salesforce. It will appear in document settings below the subject line.
When using this field your pre header should appear in the relevant field in Salesforce.
When you do this, Salesforce takes the pre header in this field and adds it to your HTML for you, so you don't have complete control of how the pre header sits in your HTML. We'd recommend having the pre header in your HTML so you have control of how it is coded.