Yes, the issue with the interface getting new prefix from the pool with every modification is a long standing one.
Good news, however, is that in 7.21 this annoying behavior will finally be addressed:
- ippool6 - take into account "subnet-id" when specified on address;
Here you can see it working fine:
Here a7 is the subnet-id (8 available bits with a /56 prefix) that you can hardcode in the address entry. With a /60 pool then of course you can only choose 4 bits.