I found this answer from a google groups search for someone who had the same problem on a different Surly.

"The issue was not the seatpost, it was the seat post clamp. I brought
the bike into my LBS and right away the mechanic thought it was the
seat post locking collar. So we found a Cross Check on the showroom
floor that had the large, super beefy Surly constrictor seat post and
put that on. That was the magic trick! I am a heavier rider (210 lbs)
so I imagine with my weight, riding mostly gravel, it needs just a
little bit more bite around the seat post. I’ve got the small seat
post collar from Surly on my LHT and have had no issues what so ever,
but that may or may not have something to do with the fact that I ride
my trucker on typically smoother surfaces that my KM."