APM displays bad page layout

If you are using APM, in a portal access mode, the APM will rewrite a few things. You can collect a web application trace, and check what the server sends and what the client receives. Here is the solution about that:

https://support.f5.com/csp/article/K13384

You could also take tcpdumps in the F5 device, an HTTP capture in the client, and check the differences.

Lastly, you can check the rewrite profile, and see if any configuration change there can help with the problem you saw in the captures.