In the scenario i've bought 2 out of the 3 legendary building for the fish farms and the forestry, both items only gave me a normal 5x charge to build the specific building.
After restarting the scenario from the menu (new game), only the last legandry building (water purifier) was in the shop, the other two missing and my building menu show a buildable amount of 0.
I used both items directly from the ship inventory. Maybe that was a mistake....