Packets that were handled by dummynet are going to the wire via
ip6_output() function. Thus it is possible, that IPv6 datagram will be
fragmented if egress interface has lower MTU.
This can be tracked by follow this path:
ip6_input -> ip6_tryforward -> pfil_run_hooks(PFIL_IN),
pfil_run_hooks(PFIL_OUT|PFIL_FWD) -> dummynet_io ->
dummynet_send -> ip6_output -> ip6_fragment