In both IPv6/4 reassembly is done only at the destination.
Can anyone provide a Reliable source or reference for this ??
I found this :https://blog.apnic.net/2016/01/28/evaluating-ipv4-and-ipv6-packet-frangmentation/
which says :
Evaluating IPv4 Packet Fragmentation
The conservative approach is for the NAT to reassemble the IP packet at the NAT, apply the NAT address transform and then pass the pack onward, fragmenting as required.
Above implies that Defragmentation can also be done at intermediate routers in case of IPv4.