Spirent TestCenter device block will respond in kind. That is if:
It receives an Neighbour Solicitation from a Link Local address then the Neighbour Advertisement will be from its Link Local address and
It receives an Neighbour Solicitation from a global address then the Neighbour Advertisement will be from its global address.
The stack does source address selection (choosing an appropriate source address to use based on the destination of a packet) when sending the Neighbour Advertisement.
From the below pcap, since the source IP for Neighbour Solicitation is a global IPv6 address, the Spirent TestCenter device responds with its global IPv6 address (2nd point).