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I encountered this bug twice – both times in games with Warlock.
I tested it in single-player – it seems the bug isn't there.
Added 5 minutes later
I tested it a little more – the bug is still alive and well!!
You need to attack neutral creatures with a weak unit and die.
And I also noticed that it only happens with upgraded units.
Can someone provide a save file with test data so we can fix it?