I’m not an L3 customer and have no idea what they may have changed. If you had taken any full backups of the old system, there are probably log files in the image that you could examine to see whether L3 behavior was different.
I do know that chan_sip will match on only the IP address specified by the host parameter; if L3 was sending from multiple IPs you would have either needed multiple chan_sip trunks or accepted the calls as guest/anonymous.
But why is this of any interest? If you understand what they require now, you are providing that, and the system is working fine, IMO you should just call it a day.
I do have backups of the old system that I could possibly look at. I was just wondering because I had to write a report for work and had to note the root cause. The root cause is that I did not take a snapshot before maintenance, but beyond that, no other config changes happened except for a reboot, so I was just thinking what could have caused this. I really appreciate all your help Stewart.