where the first two commands should ensure that no FTL process (in whatever status) is running and the last command should start pihole-FTL in debugging mode (i.e. more verbose output and no switching into daemon mode).
Thanks for letting me know! I think the issue should be resolved now.
More information:
It appears like - although your Pi-hole core was on development - you still used FTL on master. This was a problem was the current development version of Pi-hole is sending SIGTERM to FTL whenever the lists are updated. However, the master version of FTL wasn't able to catch this signal and react accordigly. Instead, it terminated as any other non-daemon software would have done as well.
I think I've found the trigger. FTL will crash when I trigger an adlist update from either command line or the web interface.
As soon as the update finishes:
Program received signal SIGHUP, Hangup.
[Thread 0xb6331460 (LWP 8907) exited]
[Thread 0xb6b31460 (LWP 8906) exited]
Program terminated with signal SIGHUP, Hangup.
The program no longer exists.
(gdb)
The FTL log shows nothing unusual still, it just stops running. So, for whatever reason pihole-FTL is not responding properly to the SIGHUP following an update.