Platform:Microsoft XBOX One
Please specify your platform model. Microsoft Xbox One X
Please select your region North America
Were you on an Expedition when the issue occurred? No
Where did this issue occur? Please include menu name or in game location. Valley of Tarsis
When did this happen? (dd.mm.yy hh:mm) 02.03.19 01:20 am
Summarize your bug Valley of Tarsis challenge. On Honor Valley Dam. Discovered in world but challenge did not activate as being discovered. Still question mark
How often does the bug occur? Every time (100%)
Steps: How can we find the bug ourselves? Not sure. Activate for all
What happens when the bug occurs? Can’t finish challenge to activate achievement
What should be happening instead? Achievement activates
Already discovered location but can’t finish challenge
Even if I stil really hope Bioware could fix this stupid bug, and (too) many others in the next update, I've found a solution: you need to create a new pilot and complete the quest with him/her because challenges are shared between pilots.
In my case, the missing landmark was the one called "workshop", but other players had the same issue in different areas, with different landmarks.
Even if I get it very early in the game, with my first pilot, I reached the location again with second one and I've completed the exploration challenge.
(Note: this solution has been suggested by another EA user I really thanks.)