This observation seems similar to Host name of client xxx => contains (at least) one invalid character at position 0 - #91 by DL6ER, where DL6ER seems to wait for user feedback.
Could you please enable DHCP logging and share DHCP related lines from /var/log/pihole/FTL.log once the issues reoccurs?
sudo pihole-FTL --config dhcp.logging true