Optional Objective Bug on 'Silgul' Mission not marked after completed

There is an optional objective on mission 'Silgul' which tell you to build 20 Hades bomber.

I've build more than 20 bomber but it have not been marked.

I've got 6 airbase (3 in the screenshot) that made 4-5 bomber per each factory already and the objective still not marked.

Although there's a dialogue saying something like "you can go after his nexus now with that overkill fleet of bombers"