Seems to just be server lag when other things are going on in the same area. You don't see it as much if you just duel someone inside a house. Though this also varies if it's near or at prime time.
Same with things like when I hit a skill and it sometimes fires twice or just doesn't register that I hit it. They refreshed the hardware but the code is still a limiting factor I think.
Yes, I experience the heavy attack animation bug when things are busy in a battleground.
Or sometimes, It even happens in PvE when I am just out harvesting materials. I have to quit the game, and sign back in to get the heavy attack animation to work right.
It is a weird, and persistent bug.