Users on one floor get a strange IPv6 prefix and slow websites

Monday morning, users in VLAN 20 on the third floor report that some websites take several seconds to start loading and a few internal tools time out. IPv4-only sites are fine. The helpdesk noticed that affected laptops have two global IPv6 addresses: one from the real prefix 2001:db8:acad:20::/64 and one from 2001:db8:77:1::/64, which nobody recognizes. The gateway is DSW1, link-local fe80::1. Access switch ASW1 has no IPv6 first hop security configured.