BQ78350-R1: Full charge capacity decrease issue

Hi Chiayuan,
It sounds like the capacity is decreasing by the FCC Learn Down limit each cycle. 256 is its default value. Something in the parameters is not set up properly, the gauge is likely learning a lower capacity than the 2500 mAh you expect.
Run a log of the charge and discharge cycles. Some good things to check:
Look at the Pending EDV value and the cell or external average cell voltage. Be sure the pending EDV is the expected value, either a fixed value if using fixed EDV or a reasonable value if you have loaded CEDV parameters.
If using the external average cell voltage for gauging, be sure you have calibrated the voltage and that it looks reasonable.
If using the lowest cell voltage, check the minimum
When the gauge reaches the EDV2 voltage it will learn, you will likely see the FullChgCap change and the RSOC jump to 7%.
You can calculate the passed charge from the elapsed time and discharge. From your 20A 3 min note this would be approximately 1 Ah, one would not expect a 2.5 Ah battery to have reached the 7% default EDV2 point to learn. If it is, the gauge will learn down each cycle as you report. The gauge will use a more accurate calculation, you want one also for comparison, the learned FCC should be about 7% (default) + the passed charge to EDV2 as described in the technical reference manual.
Of course be sure that the cells can handle the 8C rate at the 2.5Ah capacity.
Look at your total passed charge in the discharge cycle to see if it matches what you expect.
With those values again:
Check the expected (design) capacity.
Check the voltages and passed charge.
Check the settings to be sure the gauge is using the EDV you expect and the selections are as intended.
See the technical reference manual for the setting descriptions and general explanation.