Ok, there is definitely something off with battling robot guardians released from ruins.
I woke up some of them (estimated combat strength about 45) and had enough combat strength on planet to hold them, but couldn´t defeat them (only engineers and garrison troops). So I scrambled a battlegroup from sol. When it got there, the beam cruisers opened fire on the ground troops.
While the turn (5 sec) run, Aurora started to throw the following errors at me:
Error in TransferPopulation
Error 91 was generated by Aurora
Object variable of With block variable not set
This error would show 3 times, followed by a single
Error in TransferPopulation
Error 3167 was generated by DAO.Field
Datensatz ist geloescht (Basicly: Set of data is deleted)
As I couldn´t get out of the error-loop, I CTRL-Alt-Del´ed out and restarted Aurora.
According to the log, my fleet had blasted all robots to smitherine (and destroying every last building I had allready recovered).
Well, fortunately, there were some reports of stange things happening with regard to ruin-robots, so I had made a backup just before opening fire.
After some playing around, a picture seems to form.
Variant 1:
If I blow up every last building, Aurora will throw the above errors at me, the colony will vanish (including my ground forces), but my fleet remains untouched
Variant 2:
If I only blow up some of the buildings with at least one remaining, the alien popultation will surrender and I keep the colony. But for some reason, my fleet in orbit as well as my ground troops are considered to belong to this alien population the fleet will surrender to me and my ground forces simply vanish.
Note: Variant 2 happens, even if no enemy ground unit is destroyed. Destroying any building seems to be sufficient.