1. Add the activity (durations, resources, etc).
2. Add the expense line and accrual type uniform over activity.
The issue you may hit is the sequence of the order to which you are updating. Here's what I did in the test project and I understand your process be different.
While the actual cost of the expenses is "auto-compute", it will not just happen. You will want to use Apply Actuals and select the appropriate data date that corresponds to your project. Here was my test scenario....
Activity is task dependent with fixed duration going for 30 days starting on 18-JAN. I had an expense line of 10,000.00 budgeted and remaining. I applied actuals with a data date of 25-JAN. Once I did this the actual cost went to 1666.67 (% of total planned duration) and remaining went to 8333.33.
There is no bug with regard to the functionality of this feature.