If I recall correctly, due to a Bioware quirk, GM possessed reatures do not give XP (not sure, but it might also be so if the AI is disabled). Only controllig, by "selecting and clicking on the players", the XP should be handed out just fine.
Were you perhaps under the influence of a "death-ghost" effect? There are no XP gain in those situations (I think). And perhaps it was a "death-ghost" effect that accidentally lingered due to bug?
In other words, did your whole party not get XP, or was it just you?