I opened up a support ticket and they said to upgrade to FPBX 14 as it has a newer version of dahdi apparently. That will take some planning as the server is based on site and I will need to swap it out somehow.
But I did look at the logs and all I could see is that when the system was booting after the ungraceful shutdown, wanpipe would try to load itself but it doesn’t find any Sangoma cards so it doesn’t load. Whereas when the ISDN line is unplugged, wanpipe loads itself and actually finds the BRI PCIe card and everything carries on as normal. Not sure if there’s any other useful info that could be in there? Shall I attach the log?