Maneuver Node with Duna encounter completed, Duna encounter gone!

[GreenUFO13]

I had a maneuver node with a Duna encounter and "completed" the burn. However, the encounter vanished after closing out the maneuver node.

Why does this happen?

[magnemoe]

Your burn tend to generate an error since it takes time, do you get closest encounter markers? you might have to set duna as target again
try to make an new node and use pro or retrograde nodes or the radial ones this will move the closest encounter nodes, if you don't have closest intercept try to move the plane change ones. 

Experiment it tend to be cheaper to do plane change halfway to Duna but radial and pro or retro grade is cheaper shortly after launch.