We haven't been able to figure out why the ISP switches are happening yet. We set the "LOG" parameter in the ISP settings.
In "Audit Logs" in Smart Console, we discovered that we can track the switches by entering the ISP's network gateway address in the search bar. This will show us which default gateway was assigned to the FW and when.
But as far as I understand, this option was available even before the "LOG" parameter was set in the ISP settings, but we couldn't find where the logs are stored that we can view after setting this parameter.