Sand worms spontaneously appeared on map in Harkonnen level 3

[juj]

While playing Harkonnen campaign mission 3, two sand worms on the map suddenly revealed themselves (at the same time):

They were not being attacked by the enemy, and no other cues occurred. They just became visible on the map.

[juj]

Same kind of behavior occurs on other levels too, e.g. Harkonnen level 5:

Oddly on Atreides and Ordos campaigns that I finished earlier this behavior did not manifest - this is happening only while playing the Harkonnen missions.