Just had the same bug in trainer vs level 1. One of my creatures died as a result of failed mutation, and the enemy's skeleton spawned in slot 10 instead of slot 4. This was the first skeleton produced from the boneyard. The creature that died on my side was also in slot 10.
![](http://imageplay.net/img/m7Gbd82365/Capture.jpg)
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http://imageplay.net/)
Based on these 2 it would appear as if the enemy's skeletons are summoned in the next available creature slot according to the player's side--instead of the AI's side.
UPDATE: Still playing that game and each skeleton spawns the same way--in the next available creature slot on my side.