Chiming in here, I see the same behaviour on a newly installed pi-hole v6 on bookworm.
I installed v6 directly based on the comment here (but did have issues with folder ownership)
Debian GNU/Linux 12 (bookworm)"
Linux 6.6.51+rpt-rpi-v8 aarch64
Core Version is v5.18.3-528-g16af216b
Web Version is v5.21-986-gcd916b17
FTL Version is vDev-148e558
debug log with arp debug enabled as recommended above: https://tricorder.pi-hole.net/gh9nsMBB
Maybe this adds value to the "hunt"…
-edit-
removed the db and restarted ftl, I also see this:
2024-10-25 13:26:00.080 CEST [24741/T24744] ERROR: add_netDB_network_address(-1, "192.168.0.168"): Failed to step (error 19): constraint failed