Default manual network search on CM11

Hello. Since approx. beginning of April I experience serious problems using cm11. What I mean:
- in poland we have a gsm carrier called p4. They have a small own network which covers main cities. This network is used by p4 to provide gsm and internet services.
- p4, to provide gsm services outside the cities, let it users to use national roaming, using ALL networks availiable (3 diffferent carriers)
- when in national roaming, internet services are not provided at all ( only gsm)

Now the problem: I live in a city. I need internet services all the time. So till April I've used manual network search, i.e. I used to choose P4 and it worked perfectly. For several moments during day (i.e. in tube) i had no signal of P4 but it was not a problem. After a moment the phone did connect to P4 again.

BUT NOW something has changed. I choose P4 manually but when I loose signal the phone connects again not to P4 but to the network with strongest signal (usually t-mobile). And it does not return to P4. So I have NO INTERNET AT ALL TILL I'LL MANUALLY RECONNECT to P4.

What happened? Is it a bug or is it intentional change in cm11? Is there a way to manage networks the old way? I'll be grateful for help cause this situation is extremly annoying.