BG95 RaspberryPi USB and serial

I am using the BG95 in my own design with a RasperryPi, Quectel Linux drivers are installed and I have the 4 USB tty’s, the BG95 is also connected to the Hardware serial ttyAMA0 of the raspi. I can connect to ttyUSB2 und ttyAMA0 with minicom, and the modem reacts to AT commands. Than I connect to our network via PPP (over ttyUSB2) and I get a network working connection. But than I cannot communicate over ttyAMA0 with the modem. Only sometimes I get a answer on AT (OK). When I stop ppp, everything is again fine and I can communicate on both connections. Is this by design, that when a PPP connection via ttyUSB2 is up, I can not communicate via the Hardware serial? I want to read out signal strength (AT+CSQ) during a ppp connection.

I have not encountered this situation, you can also try to send AT commands with the ttyUSB3 device.