Ok, so the missing client names are not an issue with the web interface or the web server, but rather pihole-FTL not providing the information.
For comparison:
{"top_sources_blocked":{"ThinkPad-X230.fritz.box|10.0.1.31":3076,"Sony-XZ1-Compact.fritz.box|10.0.1.22":2771,"Google-Pixel-{"top_sources_blocked":{"ThinkPad-X230.fritz.box|10.0.1.31":3076,"Sony-XZ1-Compact.fritz.box|10.0.1.22":2771,"Google-Pixel-4a.fritz.box|10.0.1.26":597,"iPad.fritz.box|10.0.1.64":374,"wireguard-sony-xz-compact|10.0.40.3":1}}4a.fritz.box|10.0.1.26":597,"iPad.fritz.box|10.0.1.64":374,"wireguard-sony-xz-compact|10.0.40.3":1}}
You could try adding DEBUG_RESOLVER=true to /etc/pihole/pihole-FTL.conf and restart FTL by pihole restartdns. This should add more information regarding hostname resolution to /var/log/pihole-FTL.log