:D There were a lot of mantis in there but I killed them all (well ED-E did while I was rumaging through some desks in the other room).
I have found that when killing multiple enemies, it can take a minute or two for all the kills to register, or at least display the exp points. It could be that ED-E killed several of these things and it took a while for the points to show up. However, they should have shown up even when you were in Good Springs. Maybe they only show up while in the cell.
I am sure the game has already tallied the points, it is the display of them that takes a while.