[Bug] - Interface, Enemy Unit Cannot engage
I just killed one sov. but I can't seem to click on this guy or engage him. do I need to declare war first?

I just killed one sov. but I can't seem to click on this guy or engage him. do I need to declare war first?

I had this same problem after killing a sovereign. The dead sovereign's cities disappeared, but one unit of his that wasn't killable was left on the map just like this. I could move over it, but not attack it.
I don't know if this is related, but I have moved onto a spider and another time a troll without initiating combat. I have tried ending my turn on them, I have tried moving off of them and back on to no avail. However, the other 5 or 6 combats I had were initiated as soon as I moved on them, or they on me.
Same problem, after killing a sovereign, their old units remain in place and can't be attacked. The spiders are a little stranger.. every time I move onto one, I attack it, but when they move onto me they only 'sometimes' attack. That is, I've had them attack my sovereign sometimes, and at other times they move right through him without initiating combat, or even end their turn on top of him without attacking. But I haven't (yet) seen a spider get bugged so that it can't be attacked, not like the leftover units of a dead sovereign.
These issues seem to be related.
Had the same problem with leftover troops of a dead Sovereign. Even build a city in the same square.
I think it's easily reproductible :

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