Remaining cost is not calculated for a Cost Resource assignment in MS Project 2010 and later versions

I have a task with 4 day duration. Added a cost resource with no cost initially set.

Then, I choose 'show split' and cost then typed the following values for the assignment and pressed enter

Cost 400, Actual Cost 100.

I was expecting remaining cost to be calculated to 300, instead the cost got changed to 100. (It is working fine if I press enter immediately after entering cost and before entering the actual cost).

It is working as expected in MS Project 2007.

Again, I tried entering the below values for the same assignment.

Cost 400, Actual 50.

Now it has calculated the remaining cost: 350 and kept the cost same:400.

Actually, we are hitting this issue in a VSTO addin for MS Project 2010/2013, where it imports task and assignments data from external application. Same code is working fine in MS Project 2007.

project.Resources.Add()

.............................................

asmt.Cost = cost;
asmt.ActualCost = actCost;

Is this a bug in MS Project or am I missing anything?

Thanks,

Narmada