replacing host header

A solution was posted back in May that looks to be what you are hitting, maybe product development could confirm:

 

 

https://tech.f5.com/home/solutions/sol4942.html

 

Click here

 

 

 

Here's the text of the solution:

 

 

 

The HTTP Header Insert field of the FastHTTP profile allows you to insert an HTTP header into an HTTP request. However, the FastHTTP profile is designed for HTTP performance and only supports insertion of a static text HTTP header.

 

 

The FastHTTP profile will not perform iRule variable expansion for the HTTP Header Insert field of the FastHTTP profile; this is considered an unnecessary performance hit for the FastHTTP profile.

 

 

Note: The HTTP profile will perform iRule variable expansion for the HTTP Header Insert field of the HTTP profile.

 

 

You can use an iRule in conjunction with the FastHTTP profile for the virtual server if the FastHTTP profile must be used and an HTTP header insertion requires iRule variable expansion. The iRule will use the HTTP::header insert iRule command and the HTTP_REQUEST event.

 

 

For information about iRule creation and iRule compatibility with the FastHTTP profile, refer to the BIG-IP Configuration Guide for Local Traffic Management for your specific version. Additionally, for information about iRule functionality and assistance, refer to http://devcentral.f5.com.

 

 

F5 Networks Product Development has entered a Feature Request (CR49530) to add iRule variable expansion functionality to the FastHTTP profile. To monitor the status of this CR, refer to the BIG-IP LTM Release Notes.