Normally, a scenario is properly marked as played if it was ended with BIS_fnc_endMission. However, that is not the case with custom showcase missions, except I am doing something wrong.
What conditions have to be met in order for a Showcase mission to be marked as played?
I solved the problem. My first assumption was that BIS_fnc_endMission would automatically take care of doneKeys[] and activateKey, since I've never used that way before.
The solution was to define a key via doneKeys[] in the description.ext, and activate that key at the end of the mission with activateKey.