Subs not following 'Random Box' instructions?

[Stewy1]

G'day guys,

I was just working on a quick mission, to find a Victor III loitering in a set area.

I tried the mission twice - after searching for a long time, I turned on 'Show Truth' and saw that the sub had long left the 'set' area (once heading way north, the second time heading way south, until it beached on an island)

This is the first time I've seen this - does anyone know what the cause is?

Thanks guys,
Stew