disable force rekey

Hello, I am facing an issue with the mobile app inside osmocom-bb. I checked with gr-gsm that my telecom operator changes the Kc ( by sending auth request)  every 5 events (call or sms). But with the same sim card, I can see on Wireshark connected to the mobile app, the osmocom-bb phone received an auth request before every event. So my guess something causes the BTS to send a new auth request but only with osmocom-bb mobile app. I checked the source code, i found the option "force-rekey". Even with this option in mobile.cfg disabled I got the same issue. I saw that when enabled this option set the key sequence to 7. I tried with the latest version of osmocombb/libosmocore. Even with previous version from 2012 and last 2011 both osmocom-bb/libosmocore, different computer/ different phone (C123, c118 and c139), different cables (I use a shielded FTDI and a CP2102). Has anybody faced the same situation ? Have you an idea from where it can come from ? Thank you for your help Frank