Hello Giovanni
According to Cisco documentation, when configuring ISATAP:
The IPv6 tunnel interface must be configured with a modified EUI-64 address
because the last 32 bits in the interface identifier are constructed using the IPv4 tunnel source address.
If you were to specify the address fully, the ISATAP operation would not function.
Also, when you configure tunnel mode ipv6ip isatap on the headend, there is no need to specify the mode in which the tunnel will be functioning on the client. On the client, you also set ipv6 address autoconfig, which means that it will obtain the IPv6 address (and the mode of the tunnel) from the headend.
I hope this has been helpful!
Laz