Ahh, someone upstream is still thinking of ipv6 like it is ipv4 and they want to conserve addresses.
I know this is not a problem you can fix, but you and others might find the following interesting.
This is a good read about how people have to get their head out of the ipv4 concepts of preserving addresses space with ipv6 https://www.networkworld.com/article/2223248/the-logic-of-bad-ipv6-address-management.html
“The IANA and all the RIRs recommend using /64 subnets everywhere in your network, including on point-to-point links. But so many designers just can’t bring themselves to do it.”
Global unicast an /64 for point to point transfer networks between routers is not desirable, becasue neighbor exhaustion attacts might bring your network down.
Either use fe80::/64 (Administratively set fe80::/64 and fe80::1/64 for each routers interface)
Or use global unicast with a /127 subnetmask but still reserve a /64 for readability reasons.
Anyways: Subnet-masks between /64 and /127 are prohibitve, of course!