Issue with ground combat on a ruin and your colony.
Mars - has ruins (partially intact colony)
Started exploiting ruins.
After a while, also colonized it. As usual, occasionally opened up vaults of robotic soldiers.
This went on for a while, without issue...
Then it looked like the population surrendered and all my ground forces vanished....saw some interesting messages during all of it.
I had A LOT of troops, 4 A.I, 4 M.I., 8 Construction, 2 rep battalions, etc, I didn't lose the battle.
I didn't even noticed these messages at first.
25th March 2034 06:17:16 Sol Casualties inflicted on enemy ground unit. Estimated losses: 22%. ID: Assault Battalion #1006 (Unknown Race)
25th March 2034 06:17:16 Sol The alien population on Mars has surrendered!
25th March 2034 06:17:16 Sol Due to the loss of Mars 84.0307 has been looted by the conquering barbarians
By the time I did notice all my ground forces were gone:
28th December 2034 12:17:16 Sol Mars has moved to a political status of Subjugated - this caught my attention
I'll poke around in the Divisions table and see what I can find.
edit:
in 5.7, sometimes new double copies of colonies would be created, and ground units would be moved there. This is likely related...as no second colony was created in this 6.1 game.
edit2:
after looking in the Divisions table, the units were definitely deleted.