As the title implies, here is the bug I encountered.

1. Enemy army (Roving Clans, for completion's sake) sieged and immediately attacked one of my cities (Necrophages).

2. A Dusk Eclipse is in place.

3. Minimized combat screen before clicking anything.

4. Applied 2 Corpse Stockpiles to attacked city.

4.1. Four Battleborn spawned on city.

4.2. Battleborn can be selected, cannot be made into army (not sure if bug or not), not included in city combat (presumably, not a bug).

5. Combat screen, once opened again, does not accept imput. Can't start combat.

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Game has been saved and closed.