thanks for your answer and contribution, as always.
NCC_Permitted is disabled in all cells. After some tests made, we noticed that this phenomenon happens not with every SIM card. In few of them even if signal level was -60 or -65 dBm they roamed to the other operator (with sig level lower than -80 dBm). With the other SIM cards everything was OK, staying in homePLMN. Do you think is SIM issue?