I hit a rather large pot hole the other day and noticed a clunk when I did. It sounded like the fork bottoming out.
Today I hit a pretty big bump at speed heading up a hill and heard the same thing again.
Not to be indelicate but how much do you weigh? The front has rebound and compression damping which should both be set the same based on your weight, luggage, pillion, as described in the manual. There is no fork preload adjustment. The zip tie experiment would be a dead giveaway. Over damping should give you a hard ride but not a clunk like bottoming or tipping. The rear has compression and rebound as well as preload adjustment. Unless someone played with all of this, the damping should have been set for solo rider and the rear preload should have been all the way out (no preload).