What you were expecting to happen, and what actually happened
Merlantis currently has exactly one legendary troop, so it cannot yet reach level 6 on either task. But only the 11 troops task claims “Not achievable yet”.
Both tasks should be given the not achievable label.
How often does this happen? When did it begin happening?
Always since 3.5
I’m not certain but for the “collect x legendary troop” tasks mythic troops also count for this task just like the “have x troops upgraded to lvl 19” lvl 20 troops also count for that task.
I cannot confirm that the tasks work like that.
Suncrest has indeed only 1 legendary/mythic troop availabe.
I can verify that what @hifklak says is correct. I have Undine and am on the same power level as you, and I have both “Collect 3 weapons from Merlantis” and “Collect 2 Legendary troops from Merlantis” greened out with check marks.
I supposed they should make a note somewhere that it includes legendary troops (or higher)