Further details, I tried a reload of the seed, and did the same thing (new game).
No units after being destroyed, but some new ones appear a turn after the battle is done.
They can't be destroyed, leaving an asset there you can't take.

Load game 12. Try attacking the surrounded forces you are at war with (95,34).
A few turns later, units from the faction reappear on the same square and cannot be interacted with.

I sometimes like to start with at least one "special" character, level 4 or 5, but I can't control what they will like. Being able to create a special "revolutionary leader" at start up, and distribute some points, or even have it variable / random (just select a "majoring in" and a "minor interests ...