In the context:

service dhcpv6-server shared-network-name <text> subnet <h:h:h:h:h:h:h:h/x> prefix-delegation  start <h:h:h:h:h:h:h:h> prefix-length

The prefix-length context help and value validator incorrectly assumes we are entering "preference" configuration with valid range between 0 and 255.

So it might have been mixed with this configuration syntax during validation and for context help lookup:

service dhcpv6-server preference

The attached image evaluates different boundary values including the incorrect ones such as 129, 255, etc that are accepted by the syntax validator. When exceeding 255, a help message is printed suggesting the values between 0 and 255 which is not applicable to prefix-length.