Work Order Progress against activity

We have an issue in Applications 10, and apparently it exists also in Cloud with following settings:
1. resource group A has cost 0.00, and resource group B has cost 1.00
2. project activity is created and released
3. work order is created and connected to activity
4. WO has task Duration 10, and Resource Group B has Qty 10
5. Activity Shows Planned Hours 100 as it should (duration x qty for B = 100)
6. When WO is Released the committed hours on Activity are shown correctly as 100
7. When we report 5 hours against the resource line, the 5 hours are reported back to Activity and Activity Gets Hours Progress
8. But when hours are reported and resource group A is used (having cost 0.00), no hours are shown on Activity. The committed hours stay as 100, when the expectation was that group A (cost 0.00) should behave as group B (cost 1.00).

Any idea, why?