Re: IPv6 path MTU discovery not working (large number of retransmited TCP data packets)

The classic problem is that  ICMP6 is filtered.   But your tcpdump shows
those.
Use netstat -s before and after to see what counters are increasing, and
you'll have to code read/etc. to figure out where things are going
astray.

(Totally fair to ask if others are seeing this too, of course.)