LTM Policies only evaluated once within same HTTP-Connection?

Have you defined a default action in your LTM policy (e.g if no other criteria is met, forward to default pool)? If your LTM policy has one condition and one action, this issue is to be expected.

By minimum, you will need:

Rule: Default_action

Optionally:

If you have multiple LTM policies which all perform a similar function, try to merge them into one to avoid any issues with exectuion priorities and execution errors. In case of multiple conditions, use the "best match" strategy to ensure no more than one conditional action can occur per request.