This corruption is likely the reason. FTL cannot perform the required MAC-to-IP lookup and assumes your client belongs to the default group (so that it is not left without any blocking at all).
pihole -g -r repair only the gravity database (/etc/pihole/gravity.db). The FTL database (/etc/pihole/pihole-FTL.db) is often too large to be repairable. You can try these instructions
If it fails, you'll have to start afresh. If you don't mind loosing the query history, you can just remove the database and restart FTL (this will work instantaneously):
sudo service pihole-FTL stop
sudo rm /etc/pihole/pihole-FTL.db
sudo service pihole-FTL start