Strange behavior on "reqirep"

[Qingshan Xie]

I configured my HAProxy to use 'reqirep' to replace the Host header from site 
A.site.com to B.site.com as below, 
backend SFARM-PROXYreqirep ^Host: Host:\ B.site.com         server Proxy 
B.site.com:80
When I tested it by accessing A.site.com/xyz, HAProxy redirected it to 
B.site.com/xyz in the first time.  its log shows 302.  but on the same browser, 
I typed A.site.com/xyz, HAProxy did not redirect it and log showed 200.  It's 
reproducible.  Can someone explain it?
Thanks, Q.Xie