MKRNB : The modem is not pulling the sms from the mobile network (after a while)

Prologue: Newest Modem Firmware installed, the error occurs on different networks with different SIM,
using the "official" ReceiveSMS Sketch from the examples section.

After booting,the MKR works as expected if a SMS is sent to the device, the modem is delivering the sms.

AT+CMGL="REC UNREAD"
+CMGL: 0,"REC UNREAD","Number",,...

After about running for 30 seconds the Modem does not download the SMS waiting on the network and loops like there is no sms:

AT+CMGL="REC UNREAD"
-> OK

If you press reset, the SMS is proceeded normal like it should.
If you take the SIM out and put it into a mobile, the device receive the sms what means that the modem of the MKR1500 is not downloading the sms thus the answer for the library is looping

AT+CMGL="REC UNREAD"
OK

I discussed that issue already on github and SARA Forum but maybe someone here has a hint or a help

Thx