For postpreservation, the response page is like a 302 redirect, but 302 is/was limited to "GET" requests. The workarund was to return a whole page, which then does a "POST" to the new location (with postpresevation data in one field).
If you have slower network, where it takes time to retrieve all the postdata (maybe your postdata is large,) , or to post again to the new location (maybe the backend is slowish), then the browser gets time to show this intermediate redirect page. Often the transition is fast enough the intermediate page does not show.
Now, you can modify the contents of the page :
Configure POST Preservation - CA Single Sign-On - 12.8 - CA Technologies Documentation
You could either leave the display part as blank page (it will display presumably for 3sec) or you can add some other message more relevant to your setup.
Cheers - Mark