PPP didn't assign IPv6 address from selected pool

RouterOS General
1

RouterBoard hardware: RB850Gx2
RouterOS software: v6.36.4 (bugfix channel)

Hi, I just found a weird issue on my RouterOS.

I had setup a ppp secret office_vpn to use the vpn-profile-trust profile.
ppp_secret.png
In the vpn profile I had set this profile to use vpn-pool-default ipv6 prefix pool.
ppp_profile.png
But after this client dial in, IPv6 Pool → Used Prefixs shows this client had been assign a IPv6 prefix from another pool.
ipv6_used.png
Tried to restart the router but didn’t resolve the issue.

Any one any ideas how to solve this issue?