The supply dolyaks in WvW sometimes enter a special kind of displacement stealth mode: They will appear to walk slower both on the main screen and on the map. Trying to attack them where they SEEM to be leads to “out of range” messages, because they’re actually still walking at full speed and their real position is further along their path.
At some point near the end of the path their graphics and map icon will teleport to their real position, usually when they reach their destination.
I mostly encounter this in the borderlands with the dolyaks going from the western supply camp to the bay fortress.
/Edit: Just now I was able to reproduce it. Two dolyaks started from that camp at the same time. I attacked one, and the other (seemed to, see above) slowed down too – maybe that’s the key to recreating this bug