Mira has dropped her workers on the player's territory on the level 10

Bizzare. Will look into it on Monday.

She's an ally during that level, allies can drop minions on allied territory, looks like working as intended to me.

...

Are you insane? Why ever would she do that? What good does it achieve? They can't do anything remotely useful but just wander around aimlessly.

It's a general rule for all ally play. You can also drop your workers/minions on her territory too, in fact one of the achievements for that level practically requires you to do so.
In terms of why:

• Allies can help each other in fights
• You can create outposts/constructs, etc in places without having to wait for an imp to walk all the way there.
• They can also pick up KO'd (friendly) units and corpses and drag them back to your own rooms.

Basically not really useless in general play, probably quite useless on that level if Mira's AI doesn't make use of it properly.

This happened to me a lot back when gold rush came out.

I was playing with a gtx 240 card which could only just manage to play the game but found it hasn't happened on my new rig....not that I can tell anyway.

ZzZombo..have you tried reinstalling the game. Just to be sure?

I'm not going to reinstall a 6 GB game.

What Internet connection do you have?

So having thought on this for a while. I think that we'll leave this for now, whatever she's doing is part of the skirmish AI which is receiving a complete overhaul anyway. If she still does it after that happens then we'll want to look into it further but until then we'll leave it be.