Routeros 6.38.1 PPPOE Server with DHCPv6 PD enabled.
IPv6 pool: Prefix xxxx:yyyy::/52 Prefix length: 64
Client: Ubnt Airos6: DHCPv6 client on pppoe, Prefix Delegation with statefull DHCPv6 Server, IPv6 Prefix Length: 64 on lan.
Everything works correctly with IPv6, but on PPPOE Server i get the above error: pool6 refused acquire: bad preferred prefix! (6)
Problem is not in Mikrotik dhcpv6 server, when you set prefix length in ubnt lan interface to /64 it requests from Mikrotik /64 and that is != /60 configured in Pool, that's why error happens.
If you configure ubnt lan to request /60 and set stateless dhcp, there is no error in log, prefix is accepted and /64 is assigned to lan interface of ubnt.
For anyone wondering, we tried to downgrade the RouterOS on hEX Lite with this problem (we were running 6.43.4). The router itself didn't downgrade (consumed the ROS package - ?) but the DHCPv6 client started working. So it might be worth a shot for you till this is fixed.