If I put this in the wrong place, I apologize. Not certain how to submit a bug.
I just updated to 5.1 and pihole -c shows FTL offline.
I have looked at the code and the reason is the file /var/run/pihole-FTL.port is empty.
If you populate the file with the correct informaton, then pihole -c will work
echo 4711 > /var/run/pihole-FTL.port