URI based pool selection

Your iRule is selecting the pool based on the existence of /abc in the request. It does not alter the request itself. So forwarding to pool1 with uri /abc. Now if I understood right this path does exist on the backend server and it will do a redirect to /. Which will result as a new request. Should you actually change the uri at the BIG-IP itself instead of the backend server?

The new request may be the culprit, based on the way BIG-IP handles http requests. Do you have OneConnect on? As a default, without OneConnect profile enabled, the iRule is triggered on the first request and on subsequent requests the traffic goes to the pool selected on the first one. If you have one connect enabled, then the iRule is evaluated on all requests and the iRule will parse and select pool0. There are ways of enabling OneConnect behaviour within the iRule, too (LB::detach etc).